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.
- Add RHITestFixture with RHI_BACKEND env var support for backend selection
- Add unit tests for: Device, Buffer, Texture, SwapChain, CommandList, CommandQueue, Shader, Fence, Sampler
- Tests can run against D3D12 or OpenGL backends via RHI_BACKEND env var
- Add integration folder placeholder for future integration tests
