docs: fix threading module documentation discrepancies

- Fix include paths: use #include "Threading/..." instead of <XCEngine/Threading/...>
- Document protected ITask constructors (ITask(), ITask(TaskPriority))
- Document Callback typedef in TaskGroup
- Clarify Mutex STL-compatible methods are const
- Note GetProgress() implementation limitation (returns 0.0f)

docs/api/threading/threading.md

@@ -47,7 +47,7 @@ Threading 模块提供了一套完整的多线程编程工具，包括线程封
 ## 任务系统使用示例
 
 ```cpp
-#include <XCEngine/Threading/TaskSystem.h>
+#include "Threading/TaskSystem.h"
 
 // 初始化任务系统
 TaskSystemConfig config;