Admit a crate

Complete the requested admission under docs/adr/adr-003-12-gate-admission.md, CONTRIBUTING.md and the forward-compatibility invariants. The outcome is an implemented crate with all applicable gate evidence and one coherent admission commit. Publication follows the user's existing authorization.

Resolve the candidate, manifest, crate spec, HEAD and concurrent changes. Audit current evidence using gate-check. Implement missing work and fix failed gates within scope; a first red result is a diagnostic, not an instruction to abandon the task. A genuinely missing decision, tool or review remains a named blocker, not an invented pass.

Use an isolated checkout for any temporary workspace admission. Edit membership structurally and review the diff; do not use a shell trap that restores a shared Cargo.toml over another session's edits. Keep test-first evidence, mutation results and contract-supported N/A explanations in the crate's existing spec.