refactor: reorganize unit tests into separate folder

- Move test_*.cpp and fixtures/ to tests/RHI/D3D12/unit/
- Create unit/CMakeLists.txt with proper test configuration
- Simplify parent CMakeLists.txt to use add_subdirectory
- Integration tests remain in integration/ folder

tests/RHI/D3D12/unit/test_command_allocator.cpp

@@ -0,0 +1,45 @@
+#include "fixtures/D3D12TestFixture.h"
+
+TEST_F(D3D12TestFixture, CommandAllocator_Placeholder) {
+    ASSERT_NE(GetCommandAllocator(), nullptr);
+}
+
+TEST_F(D3D12TestFixture, CommandAllocator_Reset) {
+    ComPtr<ID3D12CommandAllocator> allocator;
+    HRESULT hr = GetDevice()->CreateCommandAllocator(
+        D3D12_COMMAND_LIST_TYPE_DIRECT,
+        IID_PPV_ARGS(&allocator)
+    );
+    ASSERT_HRESULT_SUCCEEDED(hr);
+    
+    hr = allocator->Reset();
+    ASSERT_HRESULT_SUCCEEDED(hr);
+}
+
+TEST_F(D3D12TestFixture, CommandAllocator_MultipleReset) {
+    ComPtr<ID3D12CommandAllocator> allocator;
+    HRESULT hr = GetDevice()->CreateCommandAllocator(
+        D3D12_COMMAND_LIST_TYPE_DIRECT,
+        IID_PPV_ARGS(&allocator)
+    );
+    ASSERT_HRESULT_SUCCEEDED(hr);
+    
+    for (int i = 0; i < 10; ++i) {
+        hr = allocator->Reset();
+        ASSERT_HRESULT_SUCCEEDED(hr);
+    }
+}
+
+TEST_F(D3D12TestFixture, CommandAllocator_DifferentTypes) {
+    D3D12_COMMAND_LIST_TYPE types[] = {
+        D3D12_COMMAND_LIST_TYPE_DIRECT,
+        D3D12_COMMAND_LIST_TYPE_COMPUTE,
+        D3D12_COMMAND_LIST_TYPE_COPY
+    };
+    
+    for (auto type : types) {
+        ComPtr<ID3D12CommandAllocator> allocator;
+        HRESULT hr = GetDevice()->CreateCommandAllocator(type, IID_PPV_ARGS(&allocator));
+        ASSERT_HRESULT_SUCCEEDED(hr);
+    }
+}