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/src/Resources/ResourceManager.cpp

@@ -65,12 +65,12 @@ Core::uint32 ResourceManager::GetRefCount(ResourceGUID guid) const {
 
 void ResourceManager::RegisterLoader(IResourceLoader* loader) {
     std::lock_guard lock(m_mutex);
-    m_loaders.Insert(loader->GetResourceType(), Core::MakeUnique<IResourceLoader>(loader));
+    m_loaders.Insert(loader->GetResourceType(), loader);
 }
 
 IResourceLoader* ResourceManager::GetLoader(ResourceType type) const {
     auto* it = m_loaders.Find(type);
-    return it != nullptr ? it->get() : nullptr;
+    return it != nullptr ? *it : nullptr;
 }
 
 IResourceLoader* ResourceManager::FindLoader(ResourceType type) {