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/characters/character.gd

@@ -61,5 +61,7 @@ func _on_drag_picked_up(_pos: Vector2) -> void:
 
 func _on_drag_released(pos: Vector2) -> void:
 	_is_held = false
+	if data == null or data.id.is_empty():
+		return
 	GameState.set_character_position(character_id, global_position)
 	character_placed.emit(self, global_position)