Port Bun.markdown heading IDs and autolinks

Tracks a focused Bun parity slice under #66.

Scope:

• implement Bun.markdown.html ATX heading rendering
• support heading levels 1–6 and HTML escaping
• support inline strong/code rendering used by headings
• implement normalized heading slugs and duplicate -N suffixes
• support ID-only and autolink heading modes
• activate upstream js/bun/md/md-heading-ids.test.ts
• add a dedicated native corpus regression and verify the Markdown directory

Implemented and pushed in c6eba79598ec09f2d4d054ac90182d3fcfc06dd7 (feat(test): port bun markdown heading ids).

Implementation:

• preserves and routes to the linked native Bun Markdown parser when available
• provides a functional Home fallback for ATX headings, HTML escaping, inline strong/code, Unicode-aware slugs, duplicate suffixes, IDs, and autolinks
• activates upstream md-heading-ids.test.ts and adds a dedicated Zig regression

Verification:

• exact file: 17 passed, 0 failed, 0 unsupported, 0 todo
• Markdown directory: 17 passed, 0 failed, 0 unsupported, 5 todo
• repository build and git diff --check: passed

The aggregate home_test gate remains tracked by #81 while unrelated workspace test jobs continue to exhaust host memory.

Completed on main in c6eba79598ec09f2d4d054ac90182d3fcfc06dd7.