ci: require pull request mutation smoke

[P4suta]

Summary

• generate the unified diff file required by cargo-mutants instead of passing a commit SHA to --in-diff
• restrict PR smoke mutation to the jlreq and jlreq-conformance product crates, and succeed explicitly when no Rust source changed
• run the PR smoke job inside CI and include it in the ci-required aggregate; keep weekly/manual full mutation in the Mutants workflow

Why

PR #29 exposed that cargo-mutants 27.1.0 interprets --in-diff as a file path. The old workflow supplied a SHA, so the optional smoke job failed before testing any mutants. Keeping the corrected job in CI makes future failures part of the required aggregate without changing external branch-protection settings.

Verification

• the historical PR feat: prepare jlreq 0.1.0 for release #29 Rust diff parses and lists 643 product-code mutants
• full release-check mutation remains at zero missed and zero timeout
• normal pre-push local CI passed, including coverage, design/API gates, MSRV, packaging, fuzz smoke, and the OCaml conformance gate
• actionlint, zizmor, ShellCheck, REUSE, and mutation-ledger checks pass

No crate publication, tag, release workflow, or GitHub Release is performed by this PR.