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/AudioManager.gd

@@ -1,5 +1,5 @@
 ## AudioManager — music playback with cross-fade, SFX playback, volume control.
-extends Node
+class_name AudioManager extends Node
 
 const DEFAULT_MUSIC_VOLUME: float = 0.6
 const CROSSFADE_DURATION: float = 1.0
@@ -31,13 +31,13 @@ func play_music(stream: AudioStream) -> void:
 	next_player.stream = stream
 	next_player.volume_db = linear_to_db(0.0)
 	next_player.play()
+	var prev_player: AudioStreamPlayer = _active_player
+	_active_player = next_player
 	var tween: Tween = create_tween()
 	tween.set_parallel(true)
-	tween.tween_property(_active_player, "volume_db", linear_to_db(0.0), CROSSFADE_DURATION)
+	tween.tween_property(prev_player, "volume_db", linear_to_db(0.0), CROSSFADE_DURATION)
 	tween.tween_property(next_player, "volume_db", linear_to_db(_music_volume), CROSSFADE_DURATION)
-	var prev_player: AudioStreamPlayer = _active_player
-	tween.tween_callback(prev_player.stop).set_delay(CROSSFADE_DURATION)
-	_active_player = next_player
+	tween.chain().tween_callback(prev_player.stop)
 
 
 func play_sfx(stream: AudioStream) -> void:

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"]

scripts/autoload/SaveManager.gd

@@ -12,6 +12,7 @@ func save_game() -> void:
 	var data: Dictionary = GameState.get_save_data()
 	var file: FileAccess = FileAccess.open(SAVE_PATH, FileAccess.WRITE)
 	if file == null:
+		push_error("SaveManager: cannot open save file for writing")
 		return
 	file.store_string(JSON.stringify(data))
 	file.close()
@@ -32,7 +33,9 @@ func load_game() -> void:
 
 func reset_game() -> void:
 	if FileAccess.file_exists(SAVE_PATH):
-		DirAccess.remove_absolute(SAVE_PATH)
+		var dir: DirAccess = DirAccess.open("user://")
+		if dir != null:
+			dir.remove("savegame.json")
 	GameState.apply_save_data({})