Fix 25 spec review issues across core and extensions

[gvonness-apolitical]

Summary

Comprehensive remediation of 25 issues identified during spec review, organized by priority:

P0 — Correctness (6 issues)

• Replace undefined attachment block example with link mark approach
• Add formal definitions for forms:form and forms:submit block types (field tables + schema)
• Document locatorType field on citation mark (spec-schema alignment)
• Add legal extension to extension index (available extensions, compatibility matrix, data files)
• Fix footnote number/symbol as conditional (not unconditionally required)
• Add cross-extension footnote model guidance (semantic = canonical)

P1 — Consistency (10 issues)

• Document extension mark naming convention (new Section 5.1 in content blocks)
• Add CRDT metadata to hash boundary table (excluded from content hash)
• Change resetOn from chapter/section to heading1-heading6 (schema + prose)
• Clarify Person identity field divergence (userId, keyId are additive, not replacements)
• Rename presentation numbering to numberingConfig to resolve type mismatch with core
• Add cross-reference mechanism selection guidance table
• Add formal field tables for change records and reply objects (collaboration)
• Add field tables for all 7 form field types
• Document ACL enforcement model as declarative/advisory

P2 — Quality (9 issues)

• Document glossary discovery mechanism and sort enum values
• Add semantic:term field table + plain-string rationale
• Note both CSL date formats as valid for bibliography issued field
• Add presentation extension file paths section
• Clarify forms:signature (visual) vs security extension (cryptographic)
• Document "today" as only supported relative date keyword
• Document phantom cross-reference limitations
• Renumber section 1.5a → 1.6
• Clarify Dublin Core version 1.1 vs Codex version 0.1

Test plan

• npm test — all schemas compile, all examples validate
• npm run check:sync — 65 block types synchronized
• npm run check:refs — 71 cross-references valid
• npm run check:coverage — all 8 extensions and 15 schemas covered