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/TimeSystem.gd

@@ -2,11 +2,17 @@ extends Node
 
 signal day_passed(year, month, day)
 
+const SPEEDS := [1.0, 5.0, 20.0]
+var speed_index := 0
+
 const DAY_LENGTH := 5.0
 var speed := 1.0
+var paused := false
 var _elapsed := 0.0
 
 func _process(delta: float) -> void:
+	if paused:
+		return
 	_elapsed += delta * speed
 	if _elapsed >= DAY_LENGTH:
 		_elapsed -= DAY_LENGTH
@@ -16,3 +22,19 @@ func _advance() -> void:
 	# GameState.year, GameState.month, GameState.day 推进
 	GameState.day = GameState.day + 1;
 	day_passed.emit()
+
+func cycle_speed() -> float:
+	speed_index = (speed_index + 1) % SPEEDS.size()
+	speed = SPEEDS[speed_index]
+	return speed
+
+func set_speed_index(index: int) -> void:
+	speed_index = clampi(index, 0, SPEEDS.size() - 1)
+	speed = SPEEDS[speed_index]
+
+func toggle_pause() -> bool:
+	paused = not paused
+	return paused
+
+func get_speed_label() -> String:
+	return "x%d" % int(speed)