Namespace academic cross-reference marks

[gvonnessi]

Summary

• Rename the academic cross-reference marks from bare type names to namespaced ones: theorem-ref → academic:theorem-ref, equation-ref → academic:equation-ref, algorithm-ref → academic:algorithm-ref.
• Bare extension mark names placed extension-owned identifiers in the core mark namespace. A consumer that does not load the academic schema saw an unknown bare mark and had to reject the whole document, whereas an unknown namespaced mark is ignored and rendered as plain text; the bare names also coupled the core content schema's known-mark registry to the academic extension.
• The renamed marks remain registered in the core dispatch + knownMarkTypes (so their target → Content Anchor URI validation is retained), joining the namespaced-but-registered set alongside legal:cite and presentation:footnote.
• Updated in lockstep: the canonicalizer's reference rewriter, the schema-closure vectors, the canonicalizer unit test, the document-hashing prose, the anchors/references table, and the academic documentation.
• The academic example's document id and content hash are recomputed to match the renamed marks. No other example's document id moves, and both signed manifest projections are unchanged.

Test plan

• npm test (schemas + examples) and npm run test:canonicalize pass.
• check:document-id — academic id recomputed and verified; other 10 ids unmoved.
• check:manifest-projection — both signed projections unchanged.
• check:schema-closure (212 vectors, all with teeth), check:readme-examples, check:sync, check:refs, check:coverage, check:manifest-extension-config pass.
• Seed-and-revert: a bare mark name now fails schema validation (unknown bare mark).
• All 19 gates green from a clean npm ci; npx tsc --noEmit clean; npm audit 0 high.