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/world_list.gd

@@ -135,15 +135,16 @@ func _create_world_card(world_dict: Dictionary) -> Control:
 	meta_label.add_theme_font_size_override("font_size", 12)
 	meta_label.add_theme_color_override("font_color", Color(0.5, 0.5, 0.5, 1.0))
 
-	# PC2: the only thing a world actually loads into right now is the one
-	# shared playable test level (scenes/game.tscn) -- there's no per-world
-	# content yet, so "Enter" doesn't need the world's id for anything.
-	# Real per-world level loading is a later concern once worlds actually
-	# have their own content to load.
+	# "Enter World" asks the backend to host this world (POST /join) and then
+	# loads scenes/game.tscn networked. If the backend has no game-server
+	# orchestration (a plain dev instance), it falls back to local
+	# single-player. All worlds still load the same test level -- per-world
+	# content is a later milestone.
+	var world_id := int(world_dict.get("id", 0))
 	var enter_button := Button.new()
 	enter_button.text = "Enter World"
 	enter_button.size_flags_horizontal = Control.SIZE_SHRINK_BEGIN
-	enter_button.pressed.connect(_on_enter_world_pressed)
+	enter_button.pressed.connect(_on_enter_world_pressed.bind(world_id, enter_button))
 
 	card.add_child(name_label)
 	card.add_child(desc_label)
@@ -153,5 +154,25 @@ func _create_world_card(world_dict: Dictionary) -> Control:
 	return panel
 
 
-func _on_enter_world_pressed() -> void:
-	get_tree().change_scene_to_file("res://scenes/game.tscn")
+func _on_enter_world_pressed(world_id: int, button: Button) -> void:
+	button.disabled = true
+	button.text = "Joining…"
+
+	var result := await ApiClient.join_world(world_id)
+
+	if result.ok and result.data is Dictionary:
+		var d: Dictionary = result.data
+		Session.begin(world_id, str(d.get("host", "")), int(d.get("port", 0)))
+		get_tree().change_scene_to_file("res://scenes/game.tscn")
+		return
+
+	if result.status == 503:
+		# This backend can't host game servers -- go in local single-player.
+		Session.clear()
+		get_tree().change_scene_to_file("res://scenes/game.tscn")
+		return
+
+	button.disabled = false
+	button.text = "Enter World"
+	error_label.text = "Couldn't join: %s" % result.detail
+	error_label.visible = true