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

project.godot

@@ -15,6 +15,7 @@ agp/api_base_url="http://127.0.0.1:8000"
 
 ApiClient="*res://scripts/api_client.gd"
 AvatarSystem="*res://scripts/avatar/avatar_system.gd"
+Session="*res://scripts/session.gd"
 
 [display]