PC3 Phase 2.3: networked play path in the game

- Session autoload: carries {world_id, host, port} from the world browser
  to the game scene.
- world_list.gd "Enter World": POST /api/worlds/{id}/join -> Session.begin
  -> load game.tscn. On 503 (backend has no orchestration) falls back to
  local single-player; other errors surface on the card.
- api_client.gd: join_world().
- game.gd (new, on game.tscn root): local single-player by default; when
  Session.active, connects a NetworkClient to the world's dedicated server
  and runs networked -- forwards the local Player's input each physics
  frame, snaps to the server position past a generous drift threshold
  (the Player keeps predicting locally with real collision), renders other
  players from snapshots (labelled capsules, lerped -- avatar appearance
  sync is a follow-up, the server doesn't replicate loadouts yet), and
  returns to the world list on disconnect.
- player.gd: exposes net_input (the input frame it applied) and
  apply_server_correction().
- network_client.gd: send_input() + get_own_server_state() for callers that
  do their own collision-aware prediction.
- scripts/mp_game_flow_client.gd + run_game_multiplayer_test.sh: two real
  clients each register -> join via the API -> game.tscn -> networked;
  A sees B and moves. PASS.

Verified end to end against an orchestration-enabled backend. Non-networked
smokes green (enter_world 9/9, world_list 5/5, player 15/15,
avatar_in_world 10/10, main_menu_full_flow 12/12).

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

scripts/api_client.gd

@@ -229,6 +229,13 @@ func get_avatar_loadout() -> Dictionary:
 	return await request(HTTPClient.METHOD_GET, "/api/avatar/loadout", "", "application/json", true)
 
 
+## Ask the backend for a dedicated server to host [param world_id] and return
+## its address. On success result.data is {"world_id", "host", "port"}.
+## 503 if this backend has no game-server orchestration.
+func join_world(world_id: int) -> Dictionary:
+	return await request(HTTPClient.METHOD_POST, "/api/worlds/%d/join" % world_id, "", "application/json", true)
+
+
 ## Save (replace) the current user's avatar loadout (requires auth).
 ## @param loadout A dict in the AvatarLoadout.to_json() shape
 ##                ({"base_body": String, "slots": {...}, "skin_tone"?: {...}}).