ASCII-safe heading id slugs (baked Unicode -> ASCII translit)

[dereuromark]

Summary

Ports the deterministic-fallback transliteration from djot-php #183 so auto-generated heading ids survive being shared as URL fragments. Auto-linkers (Slack, email, plain-text aggregators) routinely truncate or mis-encode non-ASCII fragments; ASCII slugs are the only fix that doesn't depend on every downstream tool getting URL detection right.

Behavior

Heading source	Slug
Café Notes	cafe-notes
Über uns	uber-uns
Привет мир	privet-mir (Cyrillic mapped)
Bob's Guide (smart quote)	bobs-guide
日本語の見出し	日本語の見出し (CJK pass-through)
Καλημέρα	καλημέρα (Greek pass-through, deliberately)

Matches carve-php's deterministic-fallback behavior byte-for-byte (both ports of the same djot-php baked map).

Why Greek/CJK/Arabic pass through

The baked map covers Latin / IPA / combining marks / Cyrillic / Latin-Extended-Additional / punctuation / super- and sub-script / currency / letterlike. Greek is deliberately excluded — its ICU transliteration is context-sensitive (αυ → au but υ → y) so it can't be baked as a context-free map. CJK / Arabic aren't in the deterministic map either. For those, authors can attach an explicit {#id} for a share-safe slug.

What's in the patch

• src/translit-map.ts — baked Unicode → ASCII map (~903 entries, mechanical port from carve-php/src/Renderer/ascii_translit_map.php).
• src/heading-ids.ts — slugify NFC-normalizes the input first (so a decomposed résumé slugs identically to its precomposed form — codex round 2), transliterates through the map, then runs the existing normalize pipeline.

Tests

• New test/heading-id-ascii.test.ts — 8 cases covering Latin diacritics, Cyrillic, smart-punct, digit-prefix, empty fallback, NFC ↔ NFD parity, Greek / CJK / Arabic pass-through.
• test/heading-ids.test.ts — pre-existing Unicode-preservation expectations updated to ASCII.
• test/corpus.test.ts — 19-heading-ids temporarily removed from IMPLEMENTED while the carve spec corpus updates its fixture from Unicode to ASCII expectations. Re-add once the carve corpus PR lands.

172 tests + 1 todo; typecheck clean; codex review clean (two real findings fixed: missing Greek-claim correction in docs, NFD normalization bug).

Coordination

This is the impl side. Companion carve repo PR to follow that updates syntax.md §4.1, the 19-heading-ids fixture, and the README Comparison cell, then bumps the spec submodule and re-vendors carve-lib.