Mutation testing

Regression tests belong in the ordinary suite; mutation runs are a bounded, opt-in way to find tests that would not notice a change.

just mutation-rust-install                                     # pinned cargo-mutants
just mutation-rust                                             # default scope
just mutation-rust --file crates/phux-core/src/window.rs --limit 16
just mutation-rust --in-diff origin/main --list

A caught mutant is evidence for that test; a missed one is a place to look, not necessarily a test to add. Unviable mutants and timeouts are not evidence of anything.

See Development for the ordinary gates.