✨ feat(mq-lang): add encode/decode builtins for text encoding conversion

[harehare]

Summary

Adds decode(bytes, label): string and encode(text, label): bytes builtins backed by encoding_rs, using WHATWG encoding labels (e.g. "shift_jis", "euc-jp", "utf-16le"). Both error on invalid byte sequences / unmappable characters rather than silently substituting, to avoid silent mojibake. Also registers the signatures in mq-check's type checker.

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

✅ 29 untouched benchmarks
⏩ 3 skipped benchmarks¹

---

_{Comparing feat/encode-decode-builtin (d96a9ff) with main (13b4392)²}

Footnotes

1. 3 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on main (a059a66) during the generation of this report, so 13b4392 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩