refactor: improve test infrastructure and fix OpenGL GLAD initialization

- Rename D3D12Enum.h to D3D12Enums.h for naming consistency
- Fix OpenGL unit test GLAD initialization by using gladLoadGL()
  instead of gladLoadGLLoader(wglGetProcAddress) for fallback support
- Migrate remaining tests to use gtest_discover_tests for granular
  test discovery (math, core, containers, memory, threading, debug,
  components, scene, resources, input, opengl)
- Remove obsolete TEST_RESOURCES_DIR and copy_directory commands
  from OpenGL unit test CMakeLists (minimal/Res doesn't exist)
- Update TEST_SPEC.md with performance metrics and per-module
  build/test commands for faster development workflow
- Update CMake path references to use lowercase paths

docs/api/threading/mutex/lock-const.md

@@ -0,0 +1,43 @@
+# Mutex::lock
+
+```cpp
+void lock() const;
+```
+
+获取互斥锁（const 版本）。如果锁已被其他线程持有，则阻塞当前线程直到锁可用。
+
+该方法是 `std::lockable` 接口的实现，可与 `std::lock_guard`、`std::unique_lock` 等 RAII 工具配合使用。
+
+**参数：** 无
+
+**返回：** 无
+
+**线程安全：** ✅
+
+**复杂度：** 平均 O(1)，最坏情况 O(n)，n 为竞争线程数
+
+**示例：**
+
+```cpp
+#include "XCEngine/Threading/Mutex.h"
+#include <mutex>
+
+XCEngine::XCEngine::Threading::Mutex mtx;
+int counter = 0;
+
+void Increment() {
+    mtx.lock();
+    ++counter;
+    mtx.unlock();
+}
+
+// 使用 RAII 自动管理锁
+void SafeIncrement() {
+    std::lock_guard<XCEngine::Threading::Mutex> guard(mtx);
+    ++counter;
+}
+```
+
+## 相关文档
+
+- [Mutex 总览](mutex.md) - 返回类总览