feat: HUD 增加时间流速控制，新增 Settings 持久化

- 顶栏改为暂停/1x/5x/20x 按钮组，当前档位高亮
- TimeSystem 支持暂停与档位切换
- 新增 Settings 单例，存档默认流速并开机应用
- GameState 补充年月日，支持 12x30 历法推进

src/Core/GameState.gd

@@ -5,9 +5,17 @@ enum GameMode { NEW_GAME, LOAD_GAME }
 var mode: GameMode = GameMode.NEW_GAME
 var save_id: String = ""
 
+# 游戏内时间（12 月 × 30 天 = 360 天/年）
+var year: int = 1
+var month: int = 1
+var day: int = 1
+
 func request_new_game() -> void:
 	mode = GameMode.NEW_GAME
 	save_id = ""
+	year = 1
+	month = 1
+	day = 1
 
 func request_load_game(id: String) -> void:
 	mode = GameMode.LOAD_GAME