✨ feat(mq-lang): add walk builtin function with tests

[harehare]

• Add walk(v, f) that recursively applies f to every node in markdown, array, and dict structures (bottom-up traversal)
• Fix dict case to use single-arg fn(entry) lambda to correctly match how with_entries passes each [key, value] pair
• Add test_walk with 24 cases covering scalar, array, dict, and markdown inputs in builtin_tests.mq

Close #1828

[codspeed-hq]

Merging this PR will not alter performance

✅ 31 untouched benchmarks

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_{Comparing feat/add-walk-builtin (f2e993a) with main (43ef138)¹}

Footnotes

1. No successful run was found on main (cebedc5) during the generation of this report, so 43ef138 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩