feat(scene-mgmt): 添加全屏图片展示功能

新增全屏/大图展示功能，支持 Yarn 命令控制：
- ShowLargeObj: 显示大图并淡入
- HideLargeObj: 隐藏大图并淡出
- HideAll: 同时隐藏大小图
- 实现大小图互斥切换逻辑
- 重构 HideObj 方法支持可选强制隐藏参数

在梦境场景中配置了大图渲染器和游戏对象，
添加相关 Yarn 命令绑定到 Dialog System。

Made-with: Cursor

Assets/Scenes/梦境.unity

@@ -482,6 +482,90 @@ MonoBehaviour:
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+--- !u!212 &762853994
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@@ -516,6 +600,7 @@ Transform:
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