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/fixtures/RHIIntegrationFixture.h

@@ -0,0 +1,55 @@
+#pragma once
+
+#include <gtest/gtest.h>
+#include <string>
+#include <windows.h>
+
+#include "XCEngine/RHI/RHIFactory.h"
+#include "XCEngine/RHI/RHIDevice.h"
+#include "XCEngine/RHI/RHICommandQueue.h"
+#include "XCEngine/RHI/RHICommandList.h"
+#include "XCEngine/RHI/RHISwapChain.h"
+#include "XCEngine/RHI/RHIScreenshot.h"
+#include "XCEngine/RHI/RHIEnums.h"
+#include "XCEngine/RHI/OpenGL/OpenGLDevice.h"
+
+namespace XCEngine {
+namespace RHI {
+namespace Integration {
+
+class RHIIntegrationFixture : public ::testing::TestWithParam<RHIType> {
+protected:
+    void SetUp() override;
+    void TearDown() override;
+
+    void WaitForGPU();
+    void BeginRender();
+    void EndRender();
+
+    RHIDevice* GetDevice() { return mDevice; }
+    RHISwapChain* GetSwapChain() { return mSwapChain; }
+    RHICommandQueue* GetCommandQueue() { return mCommandQueue; }
+    RHICommandList* GetCommandList() { return mCommandList; }
+    RHIScreenshot* GetScreenshot() { return mScreenshot; }
+    RHIType GetBackendType() const { return GetParam(); }
+    HWND GetWindowHandle() const { return mWindow; }
+    int GetCurrentBackBufferIndex() const { return mCurrentBackBufferIndex; }
+
+    virtual void RenderFrame() {}
+
+    bool TakeScreenshot(const char* filename);
+    bool CompareWithGoldenTemplate(const char* outputPpm, const char* gtPpm, float threshold = 0.0f);
+
+private:
+    RHIDevice* mDevice = nullptr;
+    RHISwapChain* mSwapChain = nullptr;
+    RHICommandQueue* mCommandQueue = nullptr;
+    RHICommandList* mCommandList = nullptr;
+    RHIScreenshot* mScreenshot = nullptr;
+    HWND mWindow = nullptr;
+    int mCurrentBackBufferIndex = 0;
+};
+
+} // namespace Integration
+} // namespace RHI
+} // namespace XCEngine