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/Settings.gd

@@ -0,0 +1,14 @@
+extends Node
+
+const SAVE_PATH := "user://settings.cfg"
+var default_speed_index := 1   # 默认 5x
+
+func load_settings() -> void:
+	var cfg := ConfigFile.new()
+	if cfg.load(SAVE_PATH) == OK:
+		default_speed_index = cfg.get_value("time", "speed_index", 1)
+
+func save_settings() -> void:
+	var cfg := ConfigFile.new()
+	cfg.set_value("time", "speed_index", default_speed_index)
+	cfg.save(SAVE_PATH)