refactor: simplify D3D12 test infrastructure

- Delete run_tests.py and run_tests.bat (use cmake/ctest directly)
- Fix integration test CMakeLists: output_ppm name, threshold=0, GT.ppm copy
- Rewrite TEST_SPEC.md to be concise
- Update integration test CTest registration

tests/RHI/D3D12/integration/minimal/CMakeLists.txt

@@ -46,6 +46,6 @@ add_test(NAME D3D12_Minimal_Integration
         $<TARGET_FILE:D3D12_Minimal>
         minimal.ppm
         ${CMAKE_CURRENT_SOURCE_DIR}/GT.ppm
-        5
+        0
     WORKING_DIRECTORY $<TARGET_FILE_DIR:D3D12_Minimal>
 )