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,

engine/src/RHI/D3D12/D3D12Screenshot.cpp

@@ -1,4 +1,7 @@
 #include "RHI/D3D12/D3D12Screenshot.h"
+#include "RHI/D3D12/D3D12Device.h"
+#include "RHI/D3D12/D3D12CommandQueue.h"
+#include "RHI/D3D12/D3D12Texture.h"
 #include "Debug/Logger.h"
 #include <d3d12.h>
 #include <stdio.h>
@@ -15,6 +18,20 @@ bool D3D12Screenshot::Capture(ID3D12Device* device,
     return CopyToReadbackAndSave(device, commandQueue, renderTarget, filename, width, height);
 }
 
+bool D3D12Screenshot::Capture(D3D12Device& device,
+                             D3D12CommandQueue& commandQueue,
+                             D3D12Texture& texture,
+                             const char* filename) {
+    return Capture(
+        device.GetDevice(),
+        commandQueue.GetCommandQueue(),
+        texture.GetResource(),
+        filename,
+        texture.GetWidth(),
+        texture.GetHeight()
+    );
+}
+
 bool D3D12Screenshot::CopyToReadbackAndSave(ID3D12Device* device,
                                             ID3D12CommandQueue* commandQueue,
                                             ID3D12Resource* renderTarget,