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.