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_queue.cpp

@@ -0,0 +1,27 @@
+#include "fixtures/D3D12TestFixture.h"
+
+TEST_F(D3D12TestFixture, CommandQueue_Placeholder) {
+    ASSERT_NE(GetCommandQueue(), nullptr);
+}
+
+TEST_F(D3D12TestFixture, CommandQueue_GetTimestampFrequency) {
+    D3D12_COMMAND_QUEUE_DESC desc = {};
+    desc.Type = D3D12_COMMAND_LIST_TYPE_DIRECT;
+    
+    ComPtr<ID3D12CommandQueue> queue;
+    HRESULT hr = GetDevice()->CreateCommandQueue(&desc, IID_PPV_ARGS(&queue));
+    ASSERT_HRESULT_SUCCEEDED(hr);
+    
+    UINT64 frequency;
+    hr = queue->GetTimestampFrequency(&frequency);
+    ASSERT_HRESULT_SUCCEEDED(hr);
+    EXPECT_GT(frequency, 0);
+}
+
+TEST_F(D3D12TestFixture, CommandQueue_ExecuteCommandLists_Empty) {
+    GetCommandList()->Reset(GetCommandAllocator(), nullptr);
+    ID3D12CommandList* commandLists[] = { GetCommandList() };
+    GetCommandQueue()->ExecuteCommandLists(1, commandLists);
+    GetCommandList()->Close();
+    WaitForGPU();
+}