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/task-group/constructor.md

@@ -0,0 +1,34 @@
+# TaskGroup::TaskGroup
+
+```cpp
+TaskGroup();
+```
+
+默认构造函数。构造一个空的任务组。
+
+**参数：** 无
+
+**返回：** 无
+
+**线程安全：** ✅
+
+**复杂度：** O(1)
+
+**注意：**
+- 构造后的任务组不包含任何任务。
+- 任务组创建后需要通过 TaskSystem::CreateTaskGroup() 实际创建。
+
+**示例：**
+
+```cpp
+#include "XCEngine/Threading/TaskGroup.h"
+
+XCEngine::Threading::TaskGroup group;
+// 使用 group 添加任务...
+```
+
+## 相关文档
+
+- [`TaskGroup`](task-group.md) - 返回类总览
+- [`AddTask`](add-task.md) - 添加任务
+- [`Wait`](wait.md) - 等待任务完成