test: Add RHI integration test framework

Add integration tests for RHI module:
- Add tests/RHI/integration/ directory with CMakeLists.txt
- Add RHIIntegrationFixture for shared test utilities
- Add minimal integration test (window creation, basic rendering)
- Add compare_ppm.py for image comparison
- Add run_integration_test.py test runner script

These integration tests verify the complete rendering pipeline
by comparing rendered output against ground truth PPM files.

tests/RHI/integration/minimal/main.cpp

@@ -0,0 +1,73 @@
+#include <windows.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+#include <gtest/gtest.h>
+
+#include "../fixtures/RHIIntegrationFixture.h"
+
+using namespace XCEngine::RHI;
+using namespace XCEngine::RHI::Integration;
+
+namespace {
+
+class MinimalTest : public RHIIntegrationFixture {
+protected:
+    void RenderFrame() override;
+};
+
+void MinimalTest::RenderFrame() {
+    RHICommandList* cmdList = GetCommandList();
+    RHICommandQueue* cmdQueue = GetCommandQueue();
+
+    cmdList->Reset();
+
+    Viewport viewport = { 0.0f, 0.0f, 1280.0f, 720.0f, 0.0f, 1.0f };
+    Rect scissorRect = { 0, 0, 1280, 720 };
+    cmdList->SetViewport(viewport);
+    cmdList->SetScissorRect(scissorRect);
+
+    float clearColor[] = { 1.0f, 0.0f, 0.0f, 1.0f };
+    cmdList->Clear(clearColor[0], clearColor[1], clearColor[2], clearColor[3], 1);
+
+    cmdList->Close();
+    void* cmdLists[] = { cmdList };
+    cmdQueue->ExecuteCommandLists(1, cmdLists);
+}
+
+TEST_P(MinimalTest, RenderClear) {
+    RHICommandQueue* cmdQueue = GetCommandQueue();
+    RHISwapChain* swapChain = GetSwapChain();
+    const int targetFrameCount = 30;
+
+    for (int frameCount = 0; frameCount <= targetFrameCount; ++frameCount) {
+        if (frameCount > 0) {
+            cmdQueue->WaitForPreviousFrame();
+        }
+
+        BeginRender();
+        RenderFrame();
+        EndRender();
+
+        if (frameCount >= targetFrameCount) {
+            cmdQueue->WaitForIdle();
+            ASSERT_TRUE(TakeScreenshot("minimal.ppm"));
+            ASSERT_TRUE(CompareWithGoldenTemplate("minimal.ppm", 
+                (GetBackendType() == RHIType::D3D12) ? "GT_D3D12.ppm" : "GT_OpenGL.ppm",
+                (GetBackendType() == RHIType::D3D12) ? 0.0f : 5.0f));
+            break;
+        }
+
+        swapChain->Present(0, 0);
+    }
+}
+
+} // namespace
+
+INSTANTIATE_TEST_SUITE_P(D3D12, MinimalTest, ::testing::Values(RHIType::D3D12));
+INSTANTIATE_TEST_SUITE_P(OpenGL, MinimalTest, ::testing::Values(RHIType::OpenGL));
+
+GTEST_API_ int main(int argc, char** argv) {
+    testing::InitGoogleTest(&argc, argv);
+    return RUN_ALL_TESTS();
+}