WIP checkpoint: Implement Godot dedicated server scene + player spawning

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docs/architecture/avatar.md

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+# Avatar Component System & glTF Attachment Spec
+
+> **Status:** Proposed
+> **Owner:** Platform team
+> **Related:** Server authority + client prediction, world chunk data format
+
+## 1. Overview
+
+This document specifies the **avatar component system** for **agp-game**, a
+Godot-based multiplayer game. It defines:
+
+- How an avatar is decomposed into **body parts** and **attachment slots**.
+- How **cosmetic attachments** (clothing, accessories, gear) mount onto the
+  base body.
+- The **glTF/GLB format requirements** that all avatar assets must satisfy so
+  they can be loaded, skinned, and networked consistently.
+
+The goal is a **modular, data-driven avatar pipeline**: a single base body rig
+plus swappable attachments, all authored in glTF and consumed by a shared Godot
+runtime. This enables player customization, cosmetic monetization, and
+server-authoritative validation of what a player is wearing.
+
+### 1.1 Design Goals
+
+- **Modularity:** Body parts and attachments are independent assets that can be
+  combined at runtime without re-exporting.
+- **Consistency:** A single, well-defined skeleton and slot convention so any
+  attachment fits any base body.
+- **Networkability:** The avatar's composition is expressed as a compact,
+  serializable **loadout** that the server can validate and replicate.
+- **Performance:** Attachments are skinned to the same skeleton and share
+  materials where possible to minimize draw calls and skinning cost.
+- **Tooling-friendly:** Assets are authored in standard glTF 2.0 and validated
+  by a build-time checker.
+
+---
+
+## 2. Avatar Decomposition
+
+### 2.1 The Base Body
+
+Every avatar is built on a **base body** — a single glTF asset containing:
+
+- A **skeleton** (the canonical rig, see §3).
+- A **base mesh** for the skin/body (head, torso, arms, legs).
+- **Attachment points** (empty nodes) that define where accessories mount.
+
+The base body is the only asset that carries the skeleton. Attachments reference
+the skeleton by **bone name** and are skinned to it; they never carry their own
+rig.
+
+### 2.2 Body Parts vs. Attachments
+
+Two distinct concepts:
+
+| Concept        | Definition                                                              | Example                          |
+|----------------|-------------------------------------------------------------------------|----------------------------------|
+| **Body part**  | A mesh that replaces part of the base body's skin (e.g., a different head, a tail, wings). | Alternate head, cybernetic arm |
+| **Attachment** | A mesh that mounts *on top of* the body without replacing it (clothing, accessories). | Shirt, hat, glasses, backpack |
+
+The distinction matters for **slot exclusivity** and **layering**:
+
+- **Body parts** are *exclusive* per slot — only one head can be equipped.
+- **Attachments** are *layered* — a hat and glasses can coexist, and both sit
+  above the head mesh.
+
+---
+
+## 3. Skeleton & Slot Convention
+
+### 3.1 Canonical Skeleton
+
+All avatars share a **canonical skeleton** with a fixed set of bone names. This
+is the contract that lets any attachment fit any body. The skeleton follows a
+humanoid convention (Godot's `Humanoid` profile / glTF VRM-style naming).
+
+Required bones (subset of the full rig):
+
+```
+Root
+├── Hips
+│   ├── Spine
+│   │   ├── Chest
+│   │   │   ├── Neck
+│   │   │   │   └── Head
+│   │   │   │       ├── HeadTop          # hat / hair mount
+│   │   │   │       ├── Jaw              # face accessories
+│   │   │   │       └── LeftEye / RightEye
+│   │   │   ├── LeftShoulder
+│   │   │   │   └── LeftUpperArm
+│   │   │   │       └── LeftLowerArm
+│   │   │   │           └── LeftHand
+│   │   │   └── RightShoulder
+│   │   │       └── RightUpperArm
+│   │   │           └── RightLowerArm
+│   │   │               └── RightHand
+│   │   └── (Chest children for torso attachments)
+│   ├── LeftUpperLeg
+│   │   └── LeftLowerLeg
+│   │       └── LeftFoot
+│   └── RightUpperLeg
+│       └── RightLowerLeg
+│           └── RightFoot
+```
+
+**Rules:**
+
+- Bone names are **case-sensitive** and must match exactly.
+- The skeleton is **T-pose** at rest for consistent skinning.
+- Attachments must be skinned to **at least** the bones they deform; static
+  accessories may be parented to a single bone (e.g., a hat parented to `Head`).
+
+### 3.2 Attachment Slots
+
+Slots are the **logical mount points** for body parts and attachments. Each slot
+maps to one or more bones and defines what can be equipped there.
+
+| Slot ID        | Category     | Mount bone(s)      | Exclusivity | Example assets                |
+|----------------|--------------|--------------------|-------------|-------------------------------|
+| `head`         | body part    | `Head`             | exclusive   | alternate head, face          |
+| `hair`         | attachment   | `HeadTop`          | exclusive   | hairstyles, hats              |
+| `face`         | attachment   | `Jaw`              | exclusive   | glasses, masks, beards        |
+| `torso`        | body part    | `Chest`            | exclusive   | alternate torso               |
+| `chest`        | attachment   | `Chest`            | exclusive   | shirts, armor, chest gear     |
+| `back`         | attachment   | `Chest` (back)     | exclusive   | backpacks, wings, capes       |
+| `left_arm`     | body part    | `LeftUpperArm`     | exclusive   | alternate arm                 |
+| `right_arm`    | body part    | `RightUpperArm`    | exclusive   | alternate arm                 |
+| `left_hand`    | attachment   | `LeftHand`         | exclusive   | gloves, weapons (held)        |
+| `right_hand`   | attachment   | `RightHand`        | exclusive   | gloves, weapons (held)        |
+| `legs`         | body part    | `Hips`             | exclusive   | alternate legs                |
+| `pants`        | attachment   | `Hips`             | exclusive   | pants, skirts                 |
+| `left_foot`    | attachment   | `LeftFoot`         | exclusive   | shoes                         |
+| `right_foot`   | attachment   | `RightFoot`        | exclusive   | shoes                         |
+| `accessory`    | attachment   | `Root` (free)      | multiple    | belts, jewelry, floating items|
+
+**Slot semantics:**
+
+- **`exclusive`** slots allow at most one equipped item at a time.
+- The **`accessory`** slot is *multi-slot* — multiple accessories may be equipped
+  simultaneously (bounded by a configurable limit, e.g., 8).
+- **Body-part slots** (`head`, `torso`, `legs`, `left_arm`, `right_arm`) replace
+  the corresponding base mesh. When a body part is equipped, the base mesh for
+  that region is hidden.
+- **Attachment slots** layer on top of the base mesh and never hide it.
+
+### 3.3 Slot → Bone Mapping
+
+Each slot declares a **primary mount bone** and an optional **secondary bone
+list** for skinning. The runtime uses this to:
+
+1. Parent static accessories to the mount bone.
+2. Skin skinned attachments to the union of the slot's bones.
+
+---
+
+## 4. glTF/GLB Format Requirements
+
+All avatar assets (base bodies, body parts, attachments) are distributed as
+**glTF 2.0** (`.gltf` + external buffers/textures) or **GLB** (`.glb`, binary
+container). GLB is the preferred distribution format for runtime loading.
+
+### 4.1 Required Conventions
+
+| Concern            | Requirement                                                                 |
+|--------------------|------------------------------------------------------------------------------|
+| **Format**         | glTF 2.0 (`.gltf` or `.glb`). GLB preferred for runtime.                     |
+| **Coordinate system** | glTF standard: **+Y up, +Z forward, right-handed**. Godot converts to its own convention on import. |
+| **Units**          | Meters. Avatar height ~1.8 m at the `Root` bone.                              |
+| **Skeleton**       | Single skeleton per asset; bone names per §3.1.                              |
+| **Skinning**       | `skins` + `nodes` with `JOINTS_0` / `WEIGHTS_0` accessors.                    |
+| **Pose**           | T-pose at rest.                                                              |
+| **Materials**      | PBR metallic-roughness. Use `KHR_materials_unlit` for emissive/UI-only items. |
+| **Textures**       | Max 2048×2048; power-of-two preferred. Use `KHR_texture_basisu` (BasisU) for runtime. |
+| **Animation**      | Optional. If present, must not rename or remove canonical bones.             |
+
+### 4.2 Asset Metadata (extras)
+
+Each asset **must** declare its slot and identity in the glTF `asset.extras`
+object. This is the machine-readable contract consumed by the build pipeline and
+runtime.
+
+```json
+{
+  "asset": {
+    "version": "2.0",
+    "extras": {
+      "agp": {
+        "type": "attachment",          // "base_body" | "body_part" | "attachment"
+        "slot": "chest",               // slot ID from §3.2
+        "id": "shirt_bandit_01",       // globally unique asset id
+        "display_name": "Bandit Shirt",
+        "version": "1.0.0",
+        "mount_bone": "Chest",         // primary mount bone
+        "skin_bones": ["Chest", "Spine", "LeftUpperArm", "RightUpperArm"],
+        "exclusive": true,             // matches slot exclusivity
+        "tags": ["cloth", "common"],
+        "author": "platform-team"
+      }
+    }
+  }
+}
+```
+
+**Validation rules for `extras.agp`:**
+
+- `type` must be one of `base_body`, `body_part`, `attachment`.
+- `slot` must be a known slot ID (§3.2).
+- `mount_bone` must exist in the canonical skeleton.
+- `skin_bones` must all exist in the canonical skeleton.
+- `exclusive` must match the slot's declared exclusivity.
+- `id` must be unique across the asset catalog.
+
+### 4.3 Node Naming Convention
+
+To make assets self-describing and mergeable, nodes follow a convention:
+
+- **Base body:** the mesh node is named `Body`; the skeleton root is `Root`.
+- **Body part:** the mesh node is named `<SlotID>_Mesh` (e.g., `head_Mesh`).
+- **Attachment:** the mesh node is named `<SlotID>_<AssetID>` (e.g.,
+  `chest_shirt_bandit_01`).
+- **Attachment points** (empty nodes) on the base body are named
+  `attach_<slot>` (e.g., `attach_head`, `attach_left_hand`).
+
+### 4.4 Material & Texture Requirements
+
+- All materials use the **metallic-roughness** PBR model.
+- Textures are embedded in the GLB or referenced as external files; external
+  references must be relative paths within the asset package.
+- **Color customization** is supported via a `baseColorFactor` tint applied at
+  runtime (multiplied over the albedo). Assets should use a neutral albedo
+  (white/gray) where tinting is intended.
+- **Masked/transparent** materials must set the correct alpha mode
+  (`MASK` or `BLEND`) and `alphaCutoff`.
+
+### 4.5 Skeleton Merging at Runtime
+
+When an attachment is loaded, the runtime **merges** it onto the base body:
+
+1. Load the base body (skeleton + base meshes).
+2. Load each equipped attachment.
+3. For each attachment, **re-target its skin** to the base body's skeleton by
+   bone name (no re-export needed).
+4. Parent static accessories to their `mount_bone`.
+5. Hide base meshes for any equipped body-part slots.
+6. Apply material tints and merge into the render tree.
+
+The merge is **name-based** — bone names must match exactly (§3.1). This is the
+core reason for the strict naming contract.
+
+---
+
+## 5. Loadout Model (Network & Persistence)
+
+The avatar's composition is serialized as a **loadout** — a compact, ordered
+list of equipped asset IDs. This is what the server validates, persists, and
+replicates.
+
+### 5.1 Loadout JSON
+
+```json
+{
+  "base_body": "base_human_male_01",
+  "slots": {
+    "head":   "head_cyber_01",
+    "hair":   "hair_mohawk_01",
+    "face":   null,
+    "torso":  null,
+    "chest":  "chest_bandit_01",
+    "back":   "backpack_01",
+    "left_arm":  null,
+    "right_arm": null,
+    "left_hand": "glove_leather_01",
+    "right_hand": "glove_leather_01",
+    "legs":   null,
+    "pants":  "pants_cargo_01",
+    "left_foot":  "shoe_boot_01",
+    "right_foot": "shoe_boot_01",
+    "accessory": ["belt_01", "necklace_01"]
+  }
+}
+```
+
+**Rules:**
+
+- `base_body` is required and must reference a valid `base_body` asset.
+- Each exclusive slot holds **at most one** asset id (or `null`).
+- `accessory` is an array (0..N, bounded).
+- The server validates that every id exists in the catalog and that slot
+  exclusivity is respected.
+
+### 5.2 Network Replication
+
+The loadout is replicated as part of the player's spawn/state data:
+
+```
+spawn_entity:
+  entity_id: uint32
+  type: player
+  transform: { x, y, rot }
+  props:
+    loadout_hash: uint32      # hash of the loadout for change detection
+    loadout: { ... }          # full loadout on spawn / on change
+```
+
+- The **full loadout** is sent on spawn and whenever it changes (reliable).
+- A **`loadout_hash`** lets clients skip re-resolving assets if unchanged.
+- The server is the **authority** on the loadout; clients send *requests* to
+  change equipment, and the server validates and broadcasts the result.
+
+### 5.3 Server-Side Validation
+
+The server rejects a loadout if:
+
+- `base_body` is missing or unknown.
+- Any slot references an unknown asset id.
+- An exclusive slot has more than one item.
+- An asset's declared `slot` does not match the slot it is placed in.
+- The accessory list exceeds the configured limit.
+- The asset is not owned/unlocked by the player (for gated cosmetics).
+
+---
+
+## 6. Godot Runtime Architecture
+
+### 6.1 Node Structure
+
+```
+Avatar (Node3D)
+├── Skeleton3D            # canonical skeleton (from base body)
+├── BodyMeshes            # base body meshes (skinned to Skeleton3D)
+│   ├── head_Mesh
+│   ├── torso_Mesh
+│   └── ...
+├── Attachments           # equipped attachments
+│   ├── chest_shirt_bandit_01
+│   ├── hair_mohawk_01
+│   └── ...
+└── AttachmentPoints      # empty nodes for dynamic mounting
+    ├── attach_head
+    ├── attach_left_hand
+    └── ...
+```
+
+### 6.2 Core Classes
+
+| Class                    | Responsibility                                              |
+|--------------------------|-------------------------------------------------------------|
+| `AvatarSystem`           | Loads base body + attachments, resolves loadouts, owns the avatar node. |
+| `AvatarLoader`           | glTF/GLB import, skeleton merge, skin re-targeting.          |
+| `AvatarSlotRegistry`     | Static registry of slot definitions (id, bones, exclusivity).|
+| `AvatarLoadout`          | Serializable loadout model + validation.                    |
+| `AvatarCatalog`          | Asset catalog (id → asset metadata + resource path).         |
+| `AvatarVisualizer`       | Builds the render tree from a loadout.                      |
+
+### 6.3 Loading Pipeline
+
+```
+loadout JSON
+   │
+   ▼
+AvatarCatalog.resolve(loadout)   # id → asset metadata + GLB path
+   │
+   ▼
+AvatarLoader.load_base_body()    # import GLB, extract skeleton + base meshes
+   │
+   ▼
+for each equipped slot:
+   AvatarLoader.load_attachment()# import GLB, re-target skin to skeleton
+   │
+   ▼
+AvatarVisualizer.assemble()      # parent, hide replaced parts, apply tints
+   │
+   ▼
+Avatar node ready for render
+```
+
+### 6.4 Performance Considerations
+
+- **Shared skeleton:** All meshes skin to one `Skeleton3D`, so skinning is
+  computed once per frame.
+- **Material batching:** Attachments that share a material are merged into a
+  single mesh where possible (static accessories).
+- **LOD:** Optional LOD variants per asset; the runtime selects based on
+  distance.
+- **Caching:** Resolved loadouts and imported GLB resources are cached by
+  `AvatarCatalog` to avoid re-importing on respawn or re-join.
+
+---
+
+## 7. Asset Pipeline & Validation
+
+### 7.1 Build-Time Validation
+
+A build-time checker validates every asset before it enters the catalog:
+
+1. **Format check:** valid glTF 2.0 / GLB.
+2. **Metadata check:** `extras.agp` present and valid (§4.2).
+3. **Skeleton check:** all `skin_bones` exist in the canonical skeleton.
+4. **Slot check:** `slot` is known and `exclusive` matches.
+5. **Mesh check:** meshes are skinned (or static accessories parented to a bone).
+6. **Texture check:** sizes, formats, and alpha modes are valid.
+7. **Uniqueness check:** `id` is unique.
+
+### 7.2 Catalog Manifest
+
+The catalog is a JSON manifest mapping asset ids to metadata and resource paths:
+
+```json
+{
+  "assets": {
+    "base_human_male_01": {
+      "type": "base_body",
+      "path": "res://assets/avatars/base_human_male_01.glb",
+      "version": "1.0.0"
+    },
+    "chest_bandit_01": {
+      "type": "attachment",
+      "slot": "chest",
+      "path": "res://assets/avatars/chest_bandit_01.glb",
+      "version": "1.0.0"
+    }
+  }
+}
+```
+
+---
+
+## 8. Security & Anti-Cheat
+
+- **Server-authoritative loadouts:** Clients only *request* equipment changes;
+  the server validates against the catalog and the player's owned items.
+- **No client asset injection:** Clients cannot send arbitrary GLB data; they
+  reference catalog ids only.
+- **Rate limiting:** Equipment-change requests are rate-limited per peer.
+- **Hash verification:** The `loadout_hash` lets the server detect tampered or
+  stale loadout state.
+
+---
+
+## 9. Open Questions / Future Work
+
+- **Morph targets / blendshapes:** Support for facial expressions and body
+  morphs (e.g., height, muscle) via glTF morph targets.
+- **Physics attachments:** Cloth simulation for capes/skirts (Godot cloth).
+- **Dynamic tinting UI:** A color-picker that writes `baseColorFactor` tints.
+- **VRM compatibility:** Consider supporting VRM (a glTF extension for
+  humanoid avatars) as an import source.
+- **Attachment LOD streaming:** Streaming high-res attachments on demand.
+
+---
+
+## 10. Summary
+
+| Concern                | Decision                                              |
+|------------------------|-------------------------------------------------------|
+| Format                 | glTF 2.0 / GLB (GLB preferred for runtime)            |
+| Skeleton               | Single canonical humanoid skeleton, name-based merge  |
+| Slots                  | Fixed slot registry; exclusive body parts + layered attachments |
+| Loadout                | Compact JSON; server-authoritative and validated      |
+| Replication            | Full loadout on spawn/change + `loadout_hash`         |
+| Runtime                | Godot `Skeleton3D` + name-based skin re-targeting     |
+| Validation             | Build-time asset checker + runtime catalog validation |

docs/architecture/networking.md

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+# Multiplayer Networking Architecture
+
+> **Status:** Proposed
+> **Owner:** Platform team
+> **Related:** Server authority + client prediction
+
+## 1. Overview
+
+This document specifies the multiplayer networking architecture for **agp-game**,
+a Godot-based game built by autonomous agents. The architecture follows a
+**server-authoritative model with client-side prediction and reconciliation**,
+which is the industry standard for fast-paced, competitive multiplayer games.
+
+The goals of this architecture are:
+
+- **Fairness & anti-cheat:** The server is the single source of truth for all
+  gameplay state. Clients never directly mutate world state.
+- **Responsiveness:** Client-side prediction hides network latency so local input
+  feels immediate.
+- **Determinism:** The server runs a fixed-timestep simulation so that state is
+  reproducible and testable.
+- **Scalability:** A clear separation between transport, simulation, and
+  replication layers allows the server to be tuned independently.
+
+---
+
+## 2. Transport Choice: ENet over WebSocket
+
+### 2.1 Decision
+
+We choose **ENet** as the primary transport for gameplay traffic.
+
+### 2.2 Justification
+
+| Criterion            | ENet                                   | WebSocket (TCP)                     |
+|----------------------|----------------------------------------|-------------------------------------|
+| **Reliability model**| Reliable + unreliable channels         | Reliable only (TCP)                 |
+| **Latency**          | Low; unreliable packets skip retransmit| Head-of-line blocking on loss       |
+| **Packet ordering**  | Per-channel ordering                   | Global strict ordering              |
+| **Congestion control**| Lightweight, game-oriented            | TCP-style (slow start, backoff)     |
+| **Godot support**    | First-class (`ENetConnection`, `MultiplayerPeerENet`) | Supported but TCP-based |
+| **Use case fit**     | Real-time action game                  | Chat, lobby, HTTP-style messaging   |
+
+**Rationale:**
+
+- **Movement & state replication** are latency-sensitive and tolerate occasional
+  packet loss. ENet's *unreliable* channel lets us send position snapshots at high
+  frequency without head-of-line blocking.
+- **RPCs that must be delivered** (e.g., interactions, spawns) use ENet's
+  *reliable* channel, giving us exactly the semantics we need without TCP's
+  retransmission stalls.
+- Godot's `MultiplayerPeerENet` is mature and integrates directly with the
+  high-level multiplayer API (`MultiplayerAPI`, `rpc()`).
+
+### 2.3 WebSocket as a Secondary Transport
+
+WebSocket remains available as a **secondary transport** for:
+
+- **Lobby / matchmaking** messaging (low-frequency, must-be-reliable).
+- **Browser builds** where raw UDP/ENet is unavailable.
+- **Admin / telemetry** channels.
+
+The transport layer is abstracted behind an interface so gameplay code does not
+depend on the underlying protocol.
+
+---
+
+## 3. Server-Side Game Loop
+
+### 3.1 Tick Rate
+
+- **Simulation tick rate:** **30 Hz** (33.33 ms per tick).
+- **Replication / snapshot rate:** **30 Hz**, aligned with the simulation tick.
+
+**Justification:**
+
+- 30 Hz is sufficient for smooth movement and interaction in a top-down / 2D
+  action game while keeping CPU and bandwidth costs predictable.
+- A fixed timestep (rather than delta-time) makes the simulation deterministic
+  and testable.
+- The tick rate is a **configuration constant** so it can be raised (e.g., to
+  60 Hz) for competitive modes without architectural changes.
+
+### 3.2 Authority Model
+
+- **Server is authoritative** for:
+  - Player position, velocity, and physics state.
+  - Health, damage, and death.
+  - Item pickups, inventory, and world-object state.
+  - Spawning and despawning of entities.
+  - Game rules, scoring, and win/lose conditions.
+- **Client is authoritative** for:
+  - Its own *input* (which keys/buttons are pressed).
+  - Local camera and UI.
+  - Cosmetic / non-gameplay presentation.
+
+### 3.3 Server Loop Pseudocode
+
+```
+while server_running:
+    process_network_inbound()      # read client inputs & RPCs
+    for _ in range(ticks_to_advance):
+        simulate_tick()            # fixed 30 Hz step
+    broadcast_snapshots()          # replicate authoritative state
+    process_network_outbound()     # flush reliable/unreliable channels
+```
+
+The server accumulates real elapsed time and advances the simulation in fixed
+`TICK_DURATION` increments, decoupling simulation from frame rate.
+
+---
+
+## 4. RPC Call Patterns
+
+RPCs are grouped by direction and reliability. All gameplay RPCs are routed
+through the high-level `MultiplayerAPI` with explicit `rpc_id` targeting.
+
+### 4.1 Client → Server (Input & Requests)
+
+| RPC                          | Channel   | Payload                                   | Notes |
+|------------------------------|-----------|-------------------------------------------|-------|
+| `request_move_input`         | unreliable| `{ seq, input_state, timestamp }`         | Batched per tick; drives prediction |
+| `request_interact`           | reliable  | `{ target_id }`                           | Open door, pick up item, use object |
+| `request_attack`             | reliable  | `{ target_id, aim_dir }`                  | Combat action |
+| `request_join`               | reliable  | `{ player_name, profile }`                | Session setup |
+| `request_respawn`            | reliable  | `{}`                                      | After death |
+
+**Design notes:**
+
+- **Input is batched** per client tick and sent with a monotonically increasing
+  `seq` so the server can detect gaps and the client can reconcile.
+- **Interactions are reliable** because they are discrete, must-happen events
+  (opening a door, picking up an item). Losing them would corrupt world state.
+
+### 4.2 Server → Client (State & Events)
+
+| RPC                          | Channel   | Payload                                   | Notes |
+|------------------------------|-----------|-------------------------------------------|-------|
+| `snapshot_state`             | unreliable| `{ tick, entities[], player_states[] }`   | Periodic authoritative state |
+| `spawn_entity`               | reliable  | `{ entity_id, type, transform, props }`   | New entity enters world |
+| `despawn_entity`             | reliable  | `{ entity_id, reason }`                   | Entity removed |
+| `interact_result`            | reliable  | `{ request_seq, success, result }`        | Acknowledges interaction |
+| `damage_event`               | reliable  | `{ target_id, amount, source_id }`        | Combat feedback |
+| `game_event`                 | reliable  | `{ type, payload }`                       | Score, win, round start/end |
+
+### 4.3 Broadcast vs. Targeted
+
+- **Broadcast (all clients):** `snapshot_state`, `spawn_entity`,
+  `despawn_entity`, `game_event`.
+- **Targeted (single client):** `interact_result`, `damage_event` (to affected
+  players), and any player-private data.
+
+---
+
+## 5. Client-Side Prediction & Reconciliation
+
+### 5.1 Prediction
+
+- The client runs a **local copy of the movement simulation** using the same
+  physics constants as the server.
+- When the player provides input, the client:
+  1. Applies the input to its local player immediately.
+  2. Renders the predicted position.
+  3. Sends the input to the server (batched, with `seq`).
+
+### 5.2 Reconciliation
+
+- The server sends authoritative `snapshot_state` at 30 Hz containing the
+  server's tick number and authoritative player positions.
+- The client keeps a **ring buffer of predicted states** keyed by input `seq`.
+- On receiving a snapshot:
+  1. Find the last predicted state that matches the server's tick.
+  2. Compute the **error** between predicted and authoritative position.
+  3. If error exceeds a threshold, **snap or smoothly interpolate** the local
+     player to the authoritative position (error correction).
+  4. Re-simulate any inputs that occurred after the server's tick (rollback).
+
+### 5.3 Interpolation of Remote Entities
+
+- **Local player:** predicted + reconciled (see above).
+- **Remote players / entities:** rendered with **interpolation delay** of
+  `2 × TICK_DURATION` (≈ 66 ms) to smooth out snapshot jitter.
+- The client buffers the last N snapshots and interpolates between them.
+
+### 5.4 Prediction Buffer Sizing
+
+- Buffer size is adaptive based on measured RTT.
+- Default: hold up to `ceil(RTT / TICK_DURATION) + 2` predicted states.
+
+---
+
+## 6. Entity & World State Replication
+
+### 6.1 Entity Model
+
+Every networked entity has:
+
+- A globally unique `entity_id` (server-assigned).
+- An `EntityType` (player, enemy, item, projectile, door, ...).
+- A `Transform` (position, rotation).
+- A typed `props` dictionary of gameplay state (health, state machine, etc.).
+
+### 6.2 Snapshot Format
+
+```
+snapshot_state:
+  tick: uint32
+  entities:
+    - entity_id: uint32
+      type: uint8
+      transform: { x, y, rot }
+      props: { ... }        # delta-encoded where possible
+  player_states:
+    - peer_id: int
+      entity_id: uint32
+      health: int
+      state: uint8
+```
+
+### 6.3 Delta Compression
+
+- Only **changed** entity fields are sent per snapshot (dirty-flag per entity).
+- Static or unchanged entities are omitted from the snapshot.
+- This keeps bandwidth proportional to *activity*, not world size.
+
+---
+
+## 7. Connection & Session Lifecycle
+
+1. **Connect:** Client opens an ENet connection to the server.
+2. **Authenticate / Join:** Client sends `request_join`; server validates and
+   assigns a `peer_id` and spawns the player entity.
+3. **Spawn:** Server broadcasts `spawn_entity` for the new player to all clients.
+4. **Play:** Input → prediction → server simulation → snapshot → reconciliation.
+5. **Disconnect:** Server removes the player entity, broadcasts `despawn_entity`,
+   and cleans up session state.
+
+---
+
+## 8. Security & Anti-Cheat
+
+- **No client-authoritative gameplay values.** Clients only send *inputs* and
+  *requests*; the server computes all outcomes.
+- **Input validation:** Server clamps movement speed, rejects out-of-bounds
+  positions, and validates interaction targets.
+- **Rate limiting:** Per-peer input and RPC rate limits prevent flooding.
+- **Sequence validation:** Gaps or replays in input `seq` are detected and
+  rejected.
+
+---
+
+## 9. Testing & Determinism
+
+- The simulation is a **pure function of (state, inputs, tick)** so it can be
+  unit-tested headlessly.
+- A **replay system** records inputs and can re-simulate to verify server and
+  client produce identical results.
+- Network code is tested with a **mock transport** that can inject latency,
+  jitter, and packet loss.
+
+---
+
+## 10. Open Questions / Future Work
+
+- **Interest management / spatial partitioning:** For large worlds, restrict
+  snapshots to entities near each client (AOI culling).
+- **Server-side physics:** Decide whether to use Godot's physics server on the
+  host or a custom deterministic physics step.
+- **Dedicated server vs. listen server:** The architecture supports both; a
+  dedicated headless server is the target for production.
+- **WebSocket fallback:** Implement the transport abstraction for browser builds.
+
+---
+
+## 11. Summary
+
+| Concern                | Decision                                        |
+|------------------------|-------------------------------------------------|
+| Transport              | ENet (primary), WebSocket (secondary)           |
+| Tick rate              | 30 Hz fixed timestep                            |
+| Authority              | Server-authoritative; client sends input only   |
+| Prediction             | Client-side prediction + reconciliation         |
+| Remote interpolation   | 2-tick interpolation delay                      |
+| Replication            | Delta-compressed snapshots at 30 Hz             |
+| Interaction RPCs       | Reliable channel                                |
+| Movement input         | Unreliable channel, batched with sequence       |

project.godot

@@ -0,0 +1,47 @@
+; Engine configuration file.
+; It's best edited using the editor UI and not directly,
+; but it can also be edited by hand.
+
+config_version=5
+
+[application]
+
+config/name="AGP Game"
+run/main_scene="res://scenes/server.tscn"
+config/features=PackedStringArray("4.6.2.stable")
+
+[autoload]
+
+Server="*res://scripts/server.gd"
+
+[display]
+
+window/size/viewport_width=1280
+window/size/viewport_height=720
+
+[input]
+
+move_forward={
+"deadzone": 0.5,
+"events": [Object(InputEventKey,"resource_local_to_scene":false,"resource_name":"","device":-1,"window_id":0,"alt_pressed":false,"shift_pressed":false,"ctrl_pressed":false,"meta_pressed":false,"pressed":false,"keycode":0,"physical_keycode":87,"key_label":0,"unicode":119,"location":0,"echo":false,"script":null)
+]
+}
+move_left={
+"deadzone": 0.5,
+"events": [Object(InputEventKey,"resource_local_to_scene":false,"resource_name":"","device":-1,"window_id":0,"alt_pressed":false,"shift_pressed":false,"ctrl_pressed":false,"meta_pressed":false,"pressed":false,"keycode":0,"physical_keycode":65,"key_label":0,"unicode":97,"location":0,"echo":false,"script":null)
+]
+}
+move_right={
+"deadzone": 0.5,
+"events": [Object(InputEventKey,"resource_local_to_scene":false,"resource_name":"","device":-1,"window_id":0,"alt_pressed":false,"shift_pressed":false,"ctrl_pressed":false,"meta_pressed":false,"pressed":false,"keycode":0,"physical_keycode":68,"key_label":0,"unicode":100,"location":0,"echo":false,"script":null)
+]
+}
+move_down={
+"deadzone": 0.5,
+"events": [Object(InputEventKey,"resource_local_to_scene":false,"resource_name":"","device":-1,"window_id":0,"alt_pressed":false,"shift_pressed":false,"ctrl_pressed":false,"meta_pressed":false,"pressed":false,"keycode":0,"physical_keycode":83,"key_label":0,"unicode":115,"location":0,"echo":false,"script":null)
+]
+}
+
+[rendering]
+
+renderer/rendering_method="forward_plus"

scenes/player.tscn

@@ -0,0 +1,6 @@
+[gd_scene load_steps=2 format=3]
+
+[ext_resource type="Script" path="res://scripts/player_spawner.gd" id="1"]
+
+[node name="Player" type="CharacterBody3D"]
+script = ExtResource("1")

scenes/server.tscn

@@ -0,0 +1,3 @@
+[gd_scene load_steps=1 format=3]
+
+[node name="Server" type="Node"]

scripts/player_spawner.gd

@@ -0,0 +1,232 @@
+class_name PlayerNode
+extends CharacterBody3D
+## PlayerNode - Server-side player entity.
+##
+## Represents a connected player on the server. Handles movement,
+## input processing, and state synchronization.
+
+# --- Signals ---
+signal health_changed(new_health: int)
+signal player_died(player_node: PlayerNode)
+
+
+# --- Constants ---
+const WALK_SPEED := 5.0
+const SPRINT_MULTIPLIER := 1.6
+const JUMP_VELOCITY := 4.5
+const GRAVITY := 9.8
+const MAX_FALL_SPEED := 20.0
+const FRICTION := 8.0
+const AIR_CONTROL := 0.3
+
+# --- Configurable ---
+@export var walk_speed: float = WALK_SPEED
+@export var sprint_multiplier: float = SPRINT_MULTIPLIER
+@export var jump_velocity: float = JUMP_VELOCITY
+@export var max_fall_speed: float = MAX_FALL_SPEED
+
+# --- State ---
+@export var max_health: int = 100
+var current_health: int = 0
+var is_alive: bool = true
+var is_sprinting: bool = false
+var peer_id: int = -1
+var player_name: String = ""
+var entity_id: int = 0
+
+# --- Input buffer ---
+var _input_queue: Array[Dictionary] = []
+var _last_processed_seq: int = -1
+
+# --- Movement ---
+var _velocity: Vector3 = Vector3.ZERO
+
+
+func _ready() -> void:
+	current_health = max_health
+	velocity = _velocity
+	# Start alive
+	is_alive = true
+
+
+func _physics_process(delta: float) -> void:
+	if not is_alive:
+		return
+	
+	# Process input from queue
+	_process_input(delta)
+	
+	# Apply gravity
+	if not is_on_floor():
+		velocity.y -= GRAVITY * delta
+		velocity.y = clampf(velocity.y, -max_fall_speed, max_fall_speed)
+	
+	# Apply friction on ground
+	if is_on_floor():
+		var move_dir := Vector2(velocity.x, velocity.z)
+		if move_dir.length() > 0.01:
+			var friction_factor := clampf(FRICTION * delta, 0.0, 1.0)
+			velocity.x = move_dir.normalized().x * move_dir.length() * (1.0 - friction_factor)
+			velocity.z = move_dir.normalized().y * move_dir.length() * (1.0 - friction_factor)
+		else:
+			velocity.x = 0.0
+			velocity.z = 0.0
+	
+	# Move and slide
+	velocity.y = clampf(velocity.y, -max_fall_speed, max_fall_speed)
+	move_and_slide()
+	
+	# Update visual position (for remote clients via RPC)
+	_update_visual_state()
+
+
+func _process_input(delta: float) -> void:
+	"""Process buffered input from the server's input queue."""
+	if _input_queue.is_empty():
+		return
+	
+	var input_data: Dictionary = _input_queue.pop_front()
+	var input_state: Dictionary = input_data.get("input_state", {})
+	
+	# Calculate movement direction
+	var direction := Vector3.ZERO
+	direction.x = input_state.get("move_x", 0.0)
+	direction.z = input_state.get("move_z", 0.0)
+	
+	# Sprint check
+	is_sprinting = input_state.get("sprint", false)
+	
+	if direction.length() > 0.01:
+		direction = direction.normalized()
+		var speed = walk_speed * sprint_multiplier if is_sprinting else walk_speed
+		velocity.x = direction.x * speed
+		velocity.z = direction.z * speed
+	else:
+		# On ground, apply friction (handled in physics_process)
+		if not is_on_floor():
+			velocity.x *= (1.0 - AIR_CONTROL * delta * 5.0)
+			velocity.z *= (1.0 - AIR_CONTROL * delta * 5.0)
+	
+	# Jump
+	if input_state.get("jump", false) and is_on_floor():
+		velocity.y = jump_velocity
+
+
+func take_damage(amount: int, source_id: int) -> Dictionary:
+	"""Take damage and return result dict."""
+	if not is_alive:
+		return {"success": false, "reason": "already_dead"}
+	
+	current_health = max(0, current_health - amount)
+	health_changed.emit(current_health)
+	
+	if current_health <= 0:
+		die()
+		return {"success": true, "health": 0, "dead": true}
+	
+	return {"success": true, "health": current_health}
+
+
+func die() -> void:
+	"""Handle player death."""
+	is_alive = false
+	velocity = Vector3.ZERO
+	player_died.emit(self)
+	
+	# Respawn after delay
+	await get_tree().create_timer(3.0).timeout
+	respawn()
+
+
+func respawn() -> void:
+	"""Respawn the player at spawn point."""
+	current_health = max_health
+	is_alive = true
+	velocity = Vector3.ZERO
+	health_changed.emit(current_health)
+	
+	# Move to spawn point (default: origin)
+	position = Vector3.ZERO
+
+
+func set_input_queue(inputs: Array[Dictionary]) -> void:
+	"""Set the input queue from the server."""
+	_input_queue = inputs
+
+
+func get_state() -> Dictionary:
+	"""Get current player state for replication."""
+	return {
+		"entity_id": entity_id,
+		"peer_id": peer_id,
+		"position": position,
+		"rotation": rotation,
+		"health": current_health,
+		"max_health": max_health,
+		"is_alive": is_alive,
+		"is_sprinting": is_sprinting,
+	}
+
+
+func get_server_state() -> Dictionary:
+	"""Get current player state for server snapshot broadcast."""
+	return get_state()
+
+
+func process_server_input(tick: int) -> void:
+	"""Called by server tick to process buffered input."""
+	# This is called from the server's _tick loop.
+	# The input queue is populated by RPCs from clients.
+	pass
+
+
+func _update_visual_state() -> void:
+	"""Update visual state (for remote interpolation)."""
+	# In a full implementation, this would update a remote player's
+	# visual representation. For now, the node itself is the authority.
+	pass
+
+
+# --- RPCs ---
+
+@rpc("any_peer", "call_remote", "reliable")
+func rpc_request_input(input_state: Dictionary, seq: int) -> void:
+	"""Receive input from client."""
+	if not is_alive:
+		return
+	# Store for processing in _physics_process
+	_input_queue.append({
+		"input_state": input_state,
+		"seq": seq,
+		"timestamp": Time.get_ticks_msec(),
+	})
+
+
+@rpc("any_peer", "call_remote", "reliable")
+func rpc_request_interact(target_id: int) -> void:
+	"""Handle interaction request from client."""
+	if not is_alive:
+		return
+	# TODO: Implement interaction logic
+	print("Player %d interacted with entity %d" % [peer_id, target_id])
+
+
+@rpc("any_peer", "call_remote", "reliable")
+func rpc_request_attack(target_id: int, aim_dir: Vector3) -> void:
+	"""Handle attack request from client."""
+	if not is_alive:
+		return
+	# TODO: Implement attack logic
+	print("Player %d attacked entity %d with aim %s" % [peer_id, target_id, aim_dir])
+
+
+@rpc("authority", "call_remote", "reliable")
+func rpc_set_position(pos: Vector3) -> void:
+	"""Server authoritative position update."""
+	position = pos
+
+
+@rpc("authority", "call_remote", "reliable")
+func rpc_set_rotation(rot: Vector3) -> void:
+	"""Server authoritative rotation update."""
+	rotation = rot

scripts/player_spawner.gd.uid

@@ -0,0 +1 @@
+uid://br27owjh2f5xd

scripts/server.gd

@@ -0,0 +1,174 @@
+## ServerMain - Dedicated server root node.
+## Accepts ENet connections, spawns Player nodes for each peer,
+## and runs the simulation tick loop at 20 Hz.
+extends Node
+
+
+## Maximum number of concurrent players.
+const MAX_PLAYERS := 16
+
+## Server tick rate (Hz).
+const TICK_RATE := 20
+
+## Tick duration in seconds.
+const TICK_DURATION := 1.0 / TICK_RATE
+
+## ENet server port.
+const SERVER_PORT := 7777
+
+## Network peer used for ENet communication.
+var network_peer: ENetMultiplayerPeer
+
+## Map of peer_id -> Player node.
+var players: Dictionary = {}
+
+## Current tick counter (server-authoritative).
+var tick: int = 0
+
+## Whether the server is running.
+var is_running: bool = false
+
+## Accumulated real time for fixed timestep simulation.
+var accumulator: float = 0.0
+
+## Player scene resource (loaded once).
+var _player_scene: PackedScene
+
+
+func _ready() -> void:
+	# Preload the player scene once.
+	_player_scene = preload("res://scenes/player.tscn")
+
+	# Create and assign the ENet server peer.
+	network_peer = _create_server_peer()
+	if network_peer:
+		multiplayer.multiplayer_peer = network_peer
+		is_running = true
+		print("[Server] Started on port %d" % SERVER_PORT)
+	else:
+		printerr("[Server] Failed to create ENet peer.")
+		get_tree().quit(1)
+
+
+# ---------------------------------------------------------------------------
+# Server creation
+# ---------------------------------------------------------------------------
+
+func _create_server_peer() -> ENetMultiplayerPeer:
+	var peer := ENetMultiplayerPeer.new()
+	var error := peer.create_server(SERVER_PORT, MAX_PLAYERS)
+	if error != OK:
+		printerr("[Server] create_server error: %d" % error)
+		return null
+	return peer
+
+
+# ---------------------------------------------------------------------------
+# Fixed-timestep simulation loop
+# ---------------------------------------------------------------------------
+
+func _process(delta: float) -> void:
+	if not is_running:
+		return
+
+	# Fixed timestep accumulator.
+	accumulator += delta
+	while accumulator >= TICK_DURATION:
+		accumulator -= TICK_DURATION
+		_tick()
+
+
+func _tick() -> void:
+	"""Main server simulation tick (called at TICK_RATE)."""
+	tick += 1
+
+	# Process player inputs and update state.
+	for peer_id in players:
+		var player: Node3D = players[peer_id]
+		if player and player.has_method("process_server_input"):
+			player.process_server_input(tick)
+
+	# Broadcast snapshot to all clients.
+	_broadcast_snapshot()
+
+
+func _broadcast_snapshot() -> void:
+	"""Broadcast the current authoritative state to all connected clients."""
+	var snapshot_data := {
+		"tick": tick,
+		"players": {},
+	}
+
+	for peer_id in players:
+		var player: Node3D = players[peer_id]
+		if player and player.has_method("get_server_state"):
+			snapshot_data.players[peer_id] = player.get_server_state()
+
+	# Send snapshot to all peers except the server itself (peer_id == 1).
+	var peer_ids: Array = multiplayer.get_peers()
+	for pid in peer_ids:
+		if pid == 1:  # 1 is the server itself
+			continue
+		rpc_id(pid, "_receive_snapshot", snapshot_data)
+
+
+# Remote: clients receive snapshots.
+@rpc("any_peer", "call_remote", "reliable")
+func _receive_snapshot(snapshot_data: Dictionary) -> void:
+	pass  # Handled by client scene.
+
+
+# ---------------------------------------------------------------------------
+# Connection lifecycle
+# ---------------------------------------------------------------------------
+
+func _on_peer_connected(peer_id: int) -> void:
+	print("[Server] Peer %d connected." % peer_id)
+
+	if players.size() >= MAX_PLAYERS:
+		print("[Server] Max players reached. Disconnecting peer %d." % peer_id)
+		rpc_id(peer_id, "_kick", "Server is full.")
+		return
+
+	# Spawn a Player node for this peer.
+	var player: Node3D = _player_scene.instantiate()
+	player.name = "Player_%d" % peer_id
+	player.peer_id = peer_id
+	add_child(player)
+	players[peer_id] = player
+
+	print("[Server] Spawned player node for peer %d." % peer_id)
+
+
+func _on_peer_disconnected(peer_id: int) -> void:
+	print("[Server] Peer %d disconnected." % peer_id)
+	if players.has(peer_id):
+		var player: Node3D = players[peer_id]
+		if player.is_inside_tree():
+			player.queue_free()
+		players.erase(peer_id)
+
+
+# ---------------------------------------------------------------------------
+# RPCs
+# ---------------------------------------------------------------------------
+
+@rpc("any_peer", "call_remote", "reliable")
+func _kick(reason: String) -> void:
+	print("[Server] Kicking caller: %s" % reason)
+	rpc_id(multiplayer.get_remote_sender_id(), "_disconnect", reason)
+
+
+@rpc("call_remote", "reliable")
+func _disconnect(reason: String) -> void:
+	print("[Server] Disconnecting: %s" % reason)
+	get_tree().quit(0)
+
+
+# ---------------------------------------------------------------------------
+# Signal connections
+# ---------------------------------------------------------------------------
+
+func _enter_tree() -> void:
+	multiplayer.peer_connected.connect(_on_peer_connected)
+	multiplayer.peer_disconnected.connect(_on_peer_disconnected)

scripts/server.gd.uid

@@ -0,0 +1 @@
+uid://cwjv826lva2gi