Mutation Check

Read docs/testing.md first.

1. Inspect the diff before running. New functions in logic modules that shell out, drive &PlatformBackend, read a TTY, or write stdout must be excluded in .cargo/mutants.toml in the same PR. Extract and test their pure decision logic. Pure helpers remain in scope.
2. Sanity-check changed exclusions with cargo mutants --list -f <file>.
3. Run a full-file sweep for each touched scoped module and library tests only: cargo mutants -f src/<file>.rs -- --lib. Redirect output to a file; do not pipe a long run through head or grep.
4. Triage mutants.out/missed.txt: add a discriminating test for real gaps, mark genuinely equivalent mutants with a narrow documented skip, and delete dead code. Confirm each new test by re-running the mutant or deliberately breaking the protected behavior.
5. Report files swept, missed count before/after, every survivor's disposition, and whether .cargo/mutants.toml changed.