Wire cargo-mutants into nightly CI for leaf crates #17

Context: Mutation testing finds tests that don't actually test anything. Too slow for per-PR; good for nightly on leaf crates.

Scope:

• Nightly job running cargo mutants scoped to: wavltree, range-tree, ecma-119, fastrand, arrayvec.
• Results posted as a summary comment to a tracking issue or as a check run.
• Surviving mutants trigger one follow-up issue each labeled mutation-survived.

Acceptance: job runs nightly; initial survivor list captured.