✨ feat(mq-test): add snapshot testing via assert_snapshot

[harehare]

Summary

Adds golden-file snapshot testing to mq-test, modeled on typst's suite/ref/store split. assert_snapshot(name, actual) is a new mq-test-only builtin that compares actual output against a checked-in __snapshots__/<file>/<name>.snap, and on mismatch writes the actual output plus a self-contained HTML diff report to .mq-test-store/ for review. --update-snapshots creates or overwrites goldens from the current output.

Type of Change

• 🐛 Bug fix
• ✨ New feature
• ♻️ Refactor
• 📝 Documentation
• ⚡ Performance
• ✅ Test
• 📦 Build / dependencies
• 👷 CI

Checklist

• I ran cargo fmt and cargo clippy and addressed any warnings
• I ran just test-all and all tests pass
• I added or updated tests covering this change
• I updated relevant documentation (/docs, crate README.md) if needed
• I added a changelog entry if this is a user-facing change

Additional Context

[codspeed-hq]

Merging this PR will not alter performance

✅ 31 untouched benchmarks

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_{Comparing feat/mq-test-snapshot-testing (311462e) with main (b51d52d)¹}

Footnotes

1. No successful run was found on main (a3b4e22) during the generation of this report, so b51d52d was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩