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/Texture.cpp

@@ -0,0 +1,40 @@
+#include "Resources/Texture.h"
+#include <cstring>
+
+namespace XCEngine {
+namespace Resources {
+
+Texture::Texture() = default;
+Texture::~Texture() = default;
+
+void Texture::Release() {
+    m_pixelData.Clear();
+    SetInvalid();
+}
+
+bool Texture::Create(Core::uint32 width, Core::uint32 height, Core::uint32 depth,
+                    Core::uint32 mipLevels, TextureType type, TextureFormat format,
+                    const void* data, size_t dataSize) {
+    m_width = width;
+    m_height = height;
+    m_depth = depth;
+    m_mipLevels = mipLevels;
+    m_textureType = type;
+    m_format = format;
+    
+    if (data && dataSize > 0) {
+        m_pixelData.Resize(dataSize);
+        std::memcpy(m_pixelData.Data(), data, dataSize);
+    }
+    
+    m_memorySize = dataSize;
+    
+    return true;
+}
+
+bool Texture::GenerateMipmaps() {
+    return false;
+}
+
+} // namespace Resources
+} // namespace XCEngine