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/objects/elevator_button.gd

@@ -20,6 +20,8 @@ func _on_input_event(_viewport: Node, event: InputEvent, _shape_idx: int) -> voi
 
 
 func _on_pressed() -> void:
+	if target_floor < 0:
+		return
 	RoomNavigator.go_to_floor(target_floor)
 	_play_bounce()