feat: Implement resource system Phase 2 - Concrete resource types

- Add Material class with shader/texture bindings and property system
- Add MaterialLoader for .mat/.json format
- Add Shader class (Vertex/Fragment/Geometry/Compute)
- Add ShaderLoader for .vert/.frag/.glsl/.hlsl
- Add AudioClip class (WAV/OGG/MP3/FLAC support)
- Add AudioLoader for audio files
- Add Texture/Mesh classes and loaders (from design doc)
- Fix HashMap iterator and String API usage
- Fix Mutex compatibility with std::lock_guard
- Update CMakeLists.txt with new resource files
- All tests pass: 11 Resources + 51 Containers

engine/include/XCEngine/Threading/Mutex.h

@@ -14,12 +14,12 @@ public:
     void Unlock() { m_mutex.unlock(); }
     bool TryLock() { return m_mutex.try_lock(); }
 
-    void lock() { Lock(); }
-    void unlock() { Unlock(); }
-    bool try_lock() { return TryLock(); }
+    void lock() const { m_mutex.lock(); }
+    void unlock() const { m_mutex.unlock(); }
+    bool try_lock() const { return m_mutex.try_lock(); }
 
 private:
-    std::mutex m_mutex;
+    mutable std::mutex m_mutex;
 };
 
 } // namespace Threading