feat: free-roam mode + fix multiplayer sync + remote player polish

Backend (ArenaRoom.js):
- Strip race state machine (lobby/countdown/playing/round/qualify). Persistent
  "playing" phase, no rounds, no checkpoints. Free-roam multi.
- Spawn lowered to y=1.5 (was 5) + MIN_DIST raised to 5 (was 3) to avoid
  ejecting overlapping players at connect.
- Schema kept intact (handshake-safe); deprecated fields default-valued.
- npm run schema:gen wired (anti-drift codegen).

Unity client:
- C# schema generated by schema-codegen into RolldSchema namespace
  (Generated/GameState.cs, Generated/Player.cs). NetworkSchema.cs removed —
  handshake no longer scans global namespace.
- NetworkManager: typed Room<GameState>, callbacks rebound, seeds players
  already in room on join.
- RemotePlayerController:
  * Post-spawn 1.5s grace window (BumpReady) — local PlayerController.HandleBump
    ignores remotes during grace.
  * Solid SphereCollider disabled during grace, re-enabled afterwards — fixes
    the kinematic-vs-dynamic eject when a new client spawns inside someone.
  * NPCBall prefab material switched from invisible-in-URP Default-Material to
    BallShader.shadergraph.
  * TrailRenderer added, tinted with player's chosen color.
  * Name label distance-scales (1x-8x) so pseudos remain readable far away.
- GameHUD: OnGUI emptied — race UI (rounds, mode, timer, playersAlive) gone.
- GameCanvas.jsx: BUILD_PREFIX/VERSION bumped for cache-bust.

Frontend WebGL build (pretty_build): final build with all the above.

game/Assets/Prefabs/NPCBall.prefab

@@ -89,7 +89,7 @@ MeshRenderer:
   m_RenderingLayerMask: 1
   m_RendererPriority: 0
   m_Materials:
-  - {fileID: 10303, guid: 0000000000000000f000000000000000, type: 0}
+  - {fileID: -876546973899608171, guid: 9535ecd79e34e1341bfe13a806935455, type: 3}
   m_StaticBatchInfo:
     firstSubMesh: 0
     subMeshCount: 0