feat: 添加截图保存Debug工具

- 添加SaveScreenshot函数使用D3D12 Readback方式读取渲染目标
- 保存为PPM格式（在第2帧自动保存到screenshot.ppm）
- 程序可自行测试渲染结果

tests/D3D12/main.cpp

@@ -721,6 +721,111 @@ void SwapD3D12Buffers() {
 	gSwapChain->Present(0, 0);
 }
 
+//=================================================================================
+// 截图保存 Debug 工具
+// 使用 Readback 方式读取渲染目标并保存为 PPM 格式
+//=================================================================================
+bool SaveScreenshot(const char* filename, int width, int height) {
+	ID3D12Device* device = gDevice.GetDevice();
+	ID3D12CommandQueue* queue = gCommandQueue.GetCommandQueue();
+	int currentRTIndex = gSwapChain->GetCurrentBackBufferIndex();
+
+	ID3D12Resource* renderTarget = gColorRTs[currentRTIndex];
+
+	D3D12_RESOURCE_DESC desc = renderTarget->GetDesc();
+
+	D3D12_HEAP_PROPERTIES heapProps = {};
+	heapProps.Type = D3D12_HEAP_TYPE_READBACK;
+	heapProps.CPUPageProperty = D3D12_CPU_PAGE_PROPERTY_WRITE_BACK;
+	heapProps.MemoryPoolPreference = D3D12_MEMORY_POOL_L0;
+
+	D3D12_RESOURCE_DESC readbackDesc = {};
+	readbackDesc.Dimension = D3D12_RESOURCE_DIMENSION_BUFFER;
+	readbackDesc.Width = width * height * 4;
+	readbackDesc.Height = 1;
+	readbackDesc.DepthOrArraySize = 1;
+	readbackDesc.MipLevels = 1;
+	readbackDesc.Format = DXGI_FORMAT_UNKNOWN;
+	readbackDesc.Layout = D3D12_TEXTURE_LAYOUT_ROW_MAJOR;
+	readbackDesc.Flags = D3D12_RESOURCE_FLAG_NONE;
+
+	ID3D12Resource* readbackBuffer = nullptr;
+	HRESULT hr = device->CreateCommittedResource(
+		&heapProps,
+		D3D12_HEAP_FLAG_NONE,
+		&readbackDesc,
+		D3D12_RESOURCE_STATE_COPY_DEST,
+		nullptr,
+		IID_PPV_ARGS(&readbackBuffer));
+
+	if (FAILED(hr)) {
+		return false;
+	}
+
+	ID3D12CommandAllocator* cmdAlloc = nullptr;
+	device->CreateCommandAllocator(D3D12_COMMAND_LIST_TYPE_DIRECT, IID_PPV_ARGS(&cmdAlloc));
+
+	ID3D12GraphicsCommandList* cmdList = nullptr;
+	device->CreateCommandList(0, D3D12_COMMAND_LIST_TYPE_DIRECT, cmdAlloc, nullptr, IID_PPV_ARGS(&cmdList));
+
+	D3D12_RESOURCE_BARRIER barrier = {};
+	barrier.Type = D3D12_RESOURCE_BARRIER_TYPE_TRANSITION;
+	barrier.Transition.pResource = renderTarget;
+	barrier.Transition.StateBefore = D3D12_RESOURCE_STATE_PRESENT;
+	barrier.Transition.StateAfter = D3D12_RESOURCE_STATE_COPY_SOURCE;
+	barrier.Transition.Subresource = D3D12_RESOURCE_BARRIER_ALL_SUBRESOURCES;
+	cmdList->ResourceBarrier(1, &barrier);
+
+	cmdList->CopyResource(readbackBuffer, renderTarget);
+
+	barrier.Transition.StateBefore = D3D12_RESOURCE_STATE_COPY_SOURCE;
+	barrier.Transition.StateAfter = D3D12_RESOURCE_STATE_PRESENT;
+	cmdList->ResourceBarrier(1, &barrier);
+
+	cmdList->Close();
+	ID3D12CommandList* ppCmdLists[] = { cmdList };
+	queue->ExecuteCommandLists(1, ppCmdLists);
+
+	gFence.Initialize(device, 0);
+	gFence.Signal(1);
+	gFence.Wait(1);
+
+	D3D12_RANGE readRange = { 0, width * height * 4 };
+	unsigned char* mappedData = nullptr;
+	readbackBuffer->Map(0, &readRange, (void**)&mappedData);
+
+	FILE* fp = fopen(filename, "wb");
+	if (!fp) {
+		readbackBuffer->Release();
+		cmdList->Release();
+		cmdAlloc->Release();
+		return false;
+	}
+
+	fprintf(fp, "P6\n%d %d\n255\n", width, height);
+	for (int y = 0; y < height; y++) {
+		for (int x = 0; x < width; x++) {
+			int idx = (y * width + x) * 4;
+			unsigned char r = mappedData[idx + 2];
+			unsigned char g = mappedData[idx + 1];
+			unsigned char b = mappedData[idx + 0];
+			fwrite(&r, 1, 1, fp);
+			fwrite(&g, 1, 1, fp);
+			fwrite(&b, 1, 1, fp);
+		}
+	}
+	fclose(fp);
+
+	D3D12_RANGE writeRange = { 0, 0 };
+	readbackBuffer->Unmap(0, &writeRange);
+
+	readbackBuffer->Release();
+	cmdList->Release();
+	cmdAlloc->Release();
+
+	return true;
+}
+
 //=================================================================================
 // 获取D3D12设备
 //=================================================================================
@@ -874,6 +979,7 @@ int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine
 	MSG msg;
 	DWORD last_time = timeGetTime();
 	DWORD appStartTime = last_time;
+	int frameCount = 0;
 	while (true) {
 		ZeroMemory(&msg, sizeof(MSG));
 		if (PeekMessage(&msg, NULL, 0, 0, PM_REMOVE)) {
@@ -883,6 +989,10 @@ int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine
 			TranslateMessage(&msg);
 			DispatchMessage(&msg);
 		} else {
+			frameCount++;
+			if (frameCount == 2) {
+				SaveScreenshot("screenshot.png", 1280, 720);
+			}
 			WaitForCompletionOfCommandList();
 			DWORD current_time = timeGetTime();
 			DWORD frameTime = current_time - last_time;