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/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({})