Code coverage

The Makefile owns the coverage commands and selected features. Install the prerequisites explicitly:

cargo install cargo-llvm-cov
rustup component add llvm-tools-preview

Then generate a report:

make coverage         # text summary
make coverage-html    # target/llvm-cov/html/
make coverage-open    # open HTML with macOS `open`; elsewhere open index.html yourself
make coverage-lcov    # target/llvm-cov/lcov.info

These targets enable all features and run library unit tests (--all-features --workspace --lib); they do not measure the separate WPT suite. All-feature builds require the corresponding backend toolchains; use cargo llvm-cov --lib --features <features> directly for a supported subset. Use the generated report for current line/region coverage and the WPT dashboard for numerical conformance. Neither metric proves that failure and ownership paths are covered.

When changing an API adapter, inspect its error, buffer-boundary and cleanup paths in addition to its successful calls. Keep coverage instrumentation separate from performance measurements.