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/player.gd

@@ -154,14 +154,23 @@ func _handle_mouse_look(event: InputEventMouseMotion) -> void:
 # Physics — gravity, jump, move_and_slide
 # ---------------------------------------------------------------------------
 
+## The input frame this player applied last physics tick, in the shape the
+## network layer sends: world-space move direction (already yaw-transformed and
+## normalized), jump edge, body yaw. scripts/game.gd reads this each frame in
+## networked mode and forwards it to the server.
+var net_input: Dictionary = {"move_x": 0.0, "move_z": 0.0, "jump": false, "look_yaw": 0.0}
+
+
 func _physics_process(delta: float) -> void:
 	# Apply gravity.
 	if not is_on_floor():
 		velocity.y -= GRAVITY * delta
 
 	# Jump.
+	var jumped := false
 	if is_on_floor() and Input.is_action_just_pressed("ui_accept"):
 		velocity.y = JUMP_VELOCITY
+		jumped = true
 
 	# Build movement direction from input. Godot's local "forward" is -Z, not
 	# +Z, so W (input.y = +1) must use -basis.z -- using +basis.z here would
@@ -173,3 +182,16 @@ func _physics_process(delta: float) -> void:
 	velocity.z = direction.z * WALK_SPEED
 
 	move_and_slide()
+
+	net_input = {
+		"move_x": direction.x, "move_z": direction.z,
+		"jump": jumped, "look_yaw": rotation.y,
+	}
+
+
+## Server-authoritative correction: called by scripts/game.gd when the local
+## predicted position has drifted too far from the server's.
+func apply_server_correction(pos: Vector3) -> void:
+	global_position = pos
+	velocity.x = 0.0
+	velocity.z = 0.0