fix(core): resolve review findings in Sprint 3-4

- Fix SaveManager reset_game to use DirAccess.remove correctly
- Add null check for FileAccess.open in save_game
- Fix AudioManager crossfade tween callback chain
- Replace fragile absolute HUD path with relative onready
- Guard character position save against empty id
- Add GameState.has_character_position helper
- Emit room_changed signal after tween completes
- Add target_floor validation in ElevatorButton
- Persist audio settings via GameState
- Add class_name to RoomNavigator, AudioManager, HUD

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/autoload/GameState.gd

@@ -7,6 +7,12 @@ signal character_moved(character_id: String, position: Vector2)
 var _character_positions: Dictionary = {}
 var _object_states: Dictionary = {}
 var current_room: String = "reception"
+var music_volume: float = 0.6
+var sfx_volume: float = 1.0
+
+
+func has_character_position(id: String) -> bool:
+	return _character_positions.has(id)
 
 
 func get_character_position(id: String) -> Vector2:
@@ -33,6 +39,8 @@ func get_save_data() -> Dictionary:
 		"character_positions": _character_positions,
 		"object_states": _object_states,
 		"current_room": current_room,
+		"music_volume": music_volume,
+		"sfx_volume": sfx_volume,
 	}
 
 
@@ -43,3 +51,7 @@ func apply_save_data(data: Dictionary) -> void:
 		_object_states = data["object_states"]
 	if data.has("current_room"):
 		current_room = data["current_room"]
+	if data.has("music_volume"):
+		music_volume = data["music_volume"]
+	if data.has("sfx_volume"):
+		sfx_volume = data["sfx_volume"]