Update API documentation and remove obsolete plan files

docs/api/math/viewport/getaspectratio.md

@@ -1,34 +1,18 @@
 # Viewport::GetAspectRatio
 
 ```cpp
-float GetAspectRatio() const;
+float GetAspectRatio() const
 ```
 
 获取视口宽高比。
 
-**返回：** 宽高比 (width / height)，如果 height 为 0 则返回 0.0f
-
-**线程安全：** ✅
+**返回：** `float` - 宽高比 (width / height)，height 为 0 时返回 0.0f
 
 **复杂度：** O(1)
 
 **示例：**
 
 ```cpp
-#include "XCEngine/Math/Rect.h"
-#include <iostream>
-
-using namespace XCEngine::Math;
-
-int main() {
-    Viewport viewport(0.0f, 0.0f, 1920.0f, 1080.0f);
-    float aspect = viewport.GetAspectRatio();
-    std::cout << "Aspect Ratio: " << aspect << "\n";
-    return 0;
-}
+Viewport viewport(0, 0, 1920, 1080);
+float aspect = viewport.GetAspectRatio();  // 1.777... (16:9)
 ```
-
-## 相关文档
-
-- [`Viewport::GetRect`](getrect.md) - 转换为 Rect
-- [Viewport 总览](viewport.md)