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

tests/Resources/test_resource_guid.cpp

@@ -1,7 +1,9 @@
 #include <gtest/gtest.h>
 #include <XCEngine/Resources/ResourceTypes.h>
+#include <XCEngine/Containers/String.h>
 
 using namespace XCEngine::Resources;
+using namespace XCEngine::Containers;
 
 namespace {
 
@@ -26,7 +28,7 @@ TEST(Resources_GUID, Generate_FromCString) {
 }
 
 TEST(Resources_GUID, Generate_FromString) {
-    Containers::String path = "models/player.fbx";
+    String path = "models/player.fbx";
     ResourceGUID guid = ResourceGUID::Generate(path);
     EXPECT_TRUE(guid.IsValid());
 }
@@ -40,9 +42,9 @@ TEST(Resources_GUID, Generate_DifferentPaths) {
 
 TEST(Resources_GUID, ToString) {
     ResourceGUID guid(0x1234567890ABCDEF);
-    Containers::String str = guid.ToString();
+    String str = guid.ToString();
     
-    EXPECT_EQ(str.Size(), 16u);
+    EXPECT_EQ(str.Length(), 16u);
 }
 
 TEST(Resources_GUID, MakeResourceGUID_Helper) {