docs: update math API docs

docs/api/math/viewport/getaspectratio.md

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+# Viewport::GetAspectRatio
+
+```cpp
+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：** ✅
+
+**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;
+}
+```
+
+## 相关文档
+
+- [`Viewport::GetRect`](getrect.md) - 转换为 Rect
+- [Viewport 总览](viewport.md)