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/main/main.gd

@@ -5,12 +5,13 @@ extends Node2D
 func _ready() -> void:
 	RoomNavigator.initialize($Camera2D)
 	SaveManager.load_game()
+	AudioManager.set_music_volume(GameState.music_volume)
+	AudioManager.set_sfx_volume(GameState.sfx_volume)
 	_apply_saved_state()
 
 
 func _apply_saved_state() -> void:
 	for character in $Characters.get_children():
 		if character is Character and character.data != null:
-			var pos: Vector2 = GameState.get_character_position(character.data.id)
-			if pos != Vector2.ZERO:
-				character.global_position = pos
+			if GameState.has_character_position(character.data.id):
+				character.global_position = GameState.get_character_position(character.data.id)