PC3 Phase 2.1: server-authoritative sim + prediction/reconciliation with replay

Made the multiplayer core actually work (it had never run a real sim):

- project.godot: physics pinned to 30 Hz -- the shared sim/snapshot/prediction rate.
- server.gd: loads the world's level (real ground + collision), spawns a
  server Player per peer at the level's PlayerSpawn, broadcasts a full
  snapshot every physics frame with each peer's state AND last_seq (the last
  input seq applied for that peer). Accepts `--port/--world/--status-file`
  args (status file = player count for a future idle reaper); defaults keep
  `godot res://scenes/server.tscn` working standalone.
- player_spawner.gd (server Player): rewrote -- drains its whole input queue
  each physics frame in seq order, integrates gravity/walk/jump with the
  shared constants, move_and_slide(), tracks last_applied_seq. No more no-op
  process_server_input().
- network_client.gd: rewrote reconciliation -- drops inputs the server has
  acked (last_seq), re-simulates the still-in-flight ones from the
  authoritative state, snaps only on real drift. Exposes get_remote_states()
  (peer_id -> latest snapshot state) for the game to render other players.
  predict_and_send() now also sends body yaw.
- proxies trimmed to the RPC set the rewrite actually uses.
- scripts/mp_test_client.gd + run_multiplayer_2p_test.sh: two real client
  processes vs one real server -- client A sees client B and its own
  movement round-trips. PASS.
- run_checkpoint_tests.py: also run the 2p shell test.

smoke_test_network_client 4/4, player 15/15, avatar_in_world 10/10, all green.

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

backend/app/config.py

@@ -80,6 +80,27 @@ class Settings(BaseSettings):
     MAX_UPLOAD_SIZE_MB: int = 100
     GLTF_MAX_FILE_SIZE_MB: int = 200
 
+    # ── Game-server orchestration ──────────────────────────────────────────
+    # The join flow (POST /api/worlds/{id}/join) ensures a headless Godot
+    # dedicated-server process exists for a world and returns its address.
+    # Disabled by default so nothing tries to spawn Godot in environments
+    # that don't have it (plain pytest, CI without the engine); the
+    # checkpoint/playtest launcher turns it on.
+    GAME_SERVERS_ENABLED: bool = False
+    # Path to the Godot 4 binary. "godot" resolves via PATH.
+    GODOT_BIN: str = str(Path.home() / "godot" / "godot")
+    # Godot project directory (the game repo root -- backend/ is a subdir).
+    GAME_PROJECT_DIR: str = str(Path(__file__).resolve().parents[2])
+    # The host clients should connect to for ENet. Defaults to the LAN IP the
+    # launcher bakes into the exe; overridable for local testing.
+    GAME_SERVER_HOST: str = "127.0.0.1"
+    # Port range for per-world dedicated servers.
+    GAME_SERVER_PORT_MIN: int = 27000
+    GAME_SERVER_PORT_MAX: int = 27099
+    # Kill a world's server this long after its last join with no active
+    # players reported.
+    GAME_SERVER_IDLE_TIMEOUT_SECONDS: int = 300
+
     model_config = SettingsConfigDict(
         env_file=".env",
         env_file_encoding="utf-8",