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: