D3D12: Add Screenshot wrapper overload and document known limitations

- Add D3D12Screenshot::Capture(D3D12Device&, D3D12CommandQueue&, D3D12Texture&, const char*) wrapper overload
- Update minimal integration test to use encapsulated APIs
- Add DescriptorHeap wrapper unit tests
- Document minimal GetBuffer native call as known limitation in TEST_SPEC.md

engine/include/XCEngine/RHI/D3D12/D3D12Screenshot.h

@@ -6,7 +6,9 @@
 namespace XCEngine {
 namespace RHI {
 
+class D3D12Device;
 class D3D12CommandQueue;
+class D3D12Texture;
 
 class D3D12Screenshot {
 public:
@@ -17,6 +19,11 @@ public:
                        uint32_t width,
                        uint32_t height);
 
+    static bool Capture(D3D12Device& device,
+                       D3D12CommandQueue& commandQueue,
+                       D3D12Texture& texture,
+                       const char* filename);
+
 private:
     static bool CopyToReadbackAndSave(ID3D12Device* device,
                                       ID3D12CommandQueue* commandQueue,