PC3 Phase 2: orchestration under checkpoint control + networking doc

- pm/checkpoint_ops.py (framework): playtest backend now launches with
  GAME_SERVERS_ENABLED + GODOT_BIN + GAME_PROJECT_DIR + GAME_SERVER_HOST so
  playtesters actually share a world.
- docs/architecture/networking.md 7a/7b: the orchestration + client-play
  design as built.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015jtxAiu5dAW4WxCNdGkrkJ

docs/architecture/networking.md

@@ -237,6 +237,62 @@ snapshot_state:
 
 ---
 
+## 7a. Server Orchestration (implemented, PC3 Phase 2)
+
+**One dedicated server process per world**, spawned on demand.
+
+```
+client                    backend (single worker)              host
+  │  POST /api/worlds/{id}/join  (bearer JWT)                    │
+  │ ───────────────────────────▶ GameServerSupervisor           │
+  │                              ensure_server(world_id):        │
+  │                                if none running for world ─── spawn:
+  │                                                              godot --headless
+  │                                                              res://scenes/server.tscn --
+  │                                                              --port <p> --world <id>
+  │                                                              --status-file <path>
+  │  ◀─────────────────────────── { host, port, world_id }       │
+  │                                                              │
+  │  ENet connect  host:port ───────────────────────────────────▶ dedicated server
+```
+
+- **Supervisor** (`backend/app/services/game_servers.py`): in-process singleton,
+  so the backend MUST run single-worker. Keys servers by `world_id`. Allocates
+  ports from `GAME_SERVER_PORT_MIN..MAX`. A background reaper kills a world's
+  server once its **status file** (`{players, tick, updated}`, rewritten by the
+  server ~1×/s) reports `players == 0` for `GAME_SERVER_IDLE_TIMEOUT_SECONDS`.
+  No crash-restart — the next join respawns.
+- **Enable flag:** `GAME_SERVERS_ENABLED` (default off). The checkpoint/playtest
+  launcher (`pm/checkpoint_ops.py`) turns it on, passes `GODOT_BIN`,
+  `GAME_PROJECT_DIR`, and `GAME_SERVER_HOST` (the LAN IP baked into the `.exe`).
+  A backend with it off returns **503** from `/join`, and the client falls back
+  to local single-player.
+- **Per-world content:** all worlds currently load the same test level (M2/M7
+  work). `world_id` still scopes a distinct process + session.
+
+### 7b. Client play path
+
+`world_list.gd` → `POST /join` → `Session.begin(world_id, host, port)` →
+load `game.tscn`. `game.gd` sees `Session.active` and connects a
+`NetworkClient`:
+
+- **Local player** keeps predicting with its own collision-aware
+  `CharacterBody3D` (`player.gd`) — that *is* the prediction. Its input frame
+  (`player.net_input`) is forwarded to the server every physics tick.
+- **Correction backstop:** if the local position drifts from the server's by
+  more than `CORRECTION_THRESHOLD` (1.5 m — generous; catches teleport / a wall
+  walked through / real desync, not prediction noise) it snaps.
+- **Remote players:** rendered from snapshots, position lerped. *Appearance sync
+  is a follow-up* — remotes currently show as labelled capsules; the server
+  does not yet replicate avatar loadouts.
+- **Disconnect** → status message → back to the world list.
+
+The `NetworkClient`'s own flat-plane prediction + input-replay reconciliation
+(§5) is exercised by `smoke_test_network_client.gd`; the collision-aware game
+path above uses the lighter `send_input()` + drift-snap instead.
+
+---
+
 ## 8. Security & Anti-Cheat
 
 - **No client-authoritative gameplay values.** Clients only send *inputs* and