introduces native llvm source-based coverage as an alternative to the
existing gcov + gcovr pipeline. driven by a new -Dcoverage_tool cache
variable (default 'gcov' for backwards compat; 'llvm' enables
-fprofile-instr-generate / -fcoverage-mapping + llvm-profdata + llvm-cov).
cmake:
- RippledSettings.cmake: add coverage_tool with validation
- RippledInterface.cmake: split coverage compile/link flags by tool
- RippledCov.cmake: dispatch to the new helper when tool=llvm
- CodeCoverageLLVM.cmake (new): setup_target_for_coverage_llvm() driving
profraw -> profdata -> export with format-aware output (lcov/json/txt/html)
ci:
- new clang-20 llvm-cov coverage matrix row (apt.llvm.org bootstrap for
clang >= 19 since 24.04 default repos cap at clang-18)
- conditional install of llvm-N vs gcovr based on coverage_tool
- artifact + codecov upload generalised to coverage.lcov | coverage.xml
- coverage cmake-args switched to *_FLAGS_DEBUG so the build action's
stdlib flag isn't clobbered
temp (revert before merging to dev):
- matrix narrowed to just the llvm-cov row on non-main refs so we can
iterate without burning runners
- 'coverage-llm' added to push trigger for direct-push CI runs
