修正 P0 UI Attribute 契约文档

docs/requirements/p0-uimanager.md

@@ -44,15 +44,10 @@ public sealed class UIPanelAttribute : Attribute
     public string Path { get; }
     public PanelStrategy? OverrideStrategy { get; }
 
-    public UIPanelAttribute(
-        string layer,
-        string path = null,
-        PanelStrategy? overrideStrategy = null)
-    {
-        Layer = layer;
-        Path = path;
-        OverrideStrategy = overrideStrategy;
-    }
+    public UIPanelAttribute(string layer);
+    public UIPanelAttribute(string layer, string path);
+    public UIPanelAttribute(string layer, PanelStrategy overrideStrategy);
+    public UIPanelAttribute(string layer, string path, PanelStrategy overrideStrategy);
 }
 ```
 
@@ -61,6 +56,7 @@ public sealed class UIPanelAttribute : Attribute
 - `Layer` 必填。
 - `Path` 可选；未填时按命名约定推导。
 - `OverrideStrategy` 可选，优先于层级默认策略。
+- 说明：C# Attribute 构造参数不支持 `Nullable<T>`，因此 `OverrideStrategy` 属性保持 nullable，但通过重载表达“未指定”和“指定覆盖策略”。
 
 ## Panel 基类