docs: 重构 API 文档结构并修正源码准确性

- 重组文档目录结构: 每个模块的概述页移动到模块子目录
- 重命名 index.md 为 main.md
- 修正所有模块文档中的错误:
  - math: FromEuler→FromEulerAngles, TransformDirection 包含缩放, Box 是 OBB, Color::ToRGBA 格式
  - containers: 新增 operator==/!= 文档, 补充 std::hash DJB 算法细节
  - core: 修复 types 链接错误
  - debug: LogLevelToString 返回大写, timestamp 是秒, Profiler 空实现标注, Windows API vs ANSI
  - memory: 修复头文件路径, malloc vs operator new, 新增方法文档
  - resources: 修复 Shader/Texture 链接错误
  - threading: TaskSystem::Wait 空实现标注, ReadWriteLock 重入描述, LambdaTask 链接
- 验证: fix_links.py 确认 0 个断裂引用

docs/api/containers/string/clear.md

@@ -0,0 +1,39 @@
+# String::Clear
+
+```cpp
+void Clear();
+```
+
+清空字符串内容，将长度设为 0，但不释放已分配的内存。
+
+**参数：** 无
+
+**返回：** 无
+
+**复杂度：** O(1)
+
+**示例：**
+```cpp
+#include "XCEngine/Containers/String.h"
+#include <iostream>
+
+int main() {
+    XCEngine::Containers::String s("Hello World");
+    std::cout << "Before clear - Length: " << s.Length()
+              << ", Capacity: " << s.Capacity() << std::endl;
+    // 输出: Before clear - Length: 11, Capacity: 12
+
+    s.Clear();
+    std::cout << "After clear - Length: " << s.Length()
+              << ", Capacity: " << s.Capacity() << std::endl;
+    // 输出: After clear - Length: 0, Capacity: 12
+    std::cout << "Empty: " << s.Empty() << std::endl;  // 输出: Empty: 1
+
+    return 0;
+}
+```
+
+## 相关文档
+
+- [String 总览](string.md) - 返回类总览
+- [Reserve / Resize](reserve-resize.md) - 内存管理