feat: 新增 TimeSystem 时间系统并注册 Autoload

- project.godot 注册 GameState/TimeSystem 单例
- 存档面板新增快速存档按钮组
- 新增 export_presets.cfg 导出配置

src/Core/TimeSystem.gd

@@ -0,0 +1,18 @@
+extends Node
+
+signal day_passed(year, month, day)
+
+const DAY_LENGTH := 5.0
+var speed := 1.0
+var _elapsed := 0.0
+
+func _process(delta: float) -> void:
+	_elapsed += delta * speed
+	if _elapsed >= DAY_LENGTH:
+		_elapsed -= DAY_LENGTH
+		_advance()
+
+func _advance() -> void:
+	# GameState.year, GameState.month, GameState.day 推进
+	GameState.day = GameState.day + 1;
+	day_passed.emit()