Add missing schemas, extension examples, and documentation improvements

[gvonness-apolitical]

Summary

This PR addresses consistency and completeness issues in the Codex specification:

• 3 new schema files for extensions that had README docs but no schemas (collaboration, forms, security)
• 5 new example documents demonstrating extension usage (academic, semantic, forms, collaboration, phantoms)
• Standardized Person/author objects across extensions using schema composition
• Extension marks integration into core content schema for strict validation
• Documentation improvements including terminology glossary and compatibility matrix

Changes by Phase

Phase 1: Missing Schema Files

• schemas/collaboration.schema.json - Comments, changes, CRDT metadata
• schemas/forms.schema.json - Form blocks, validation, conditional validation
• schemas/security.schema.json - Signatures, encryption, access control

Phase 2: Extension Marks Integration

• Updated content.schema.json to reference semantic/academic extension marks
• Updated validation scripts to handle schema dependencies

Phase 3: Author/Person Standardization

• Added base person definition with identifier field to anchor.schema.json
• Updated phantoms, collaboration, security schemas to use allOf composition
• Added ORCID recommendation for academic documents

Phase 4: Extension Examples

• examples/academic-document/ - Theorems, proofs, exercises, algorithms
• examples/semantic-document/ - Citations, footnotes, glossary, entities
• examples/forms-document/ - Comprehensive form with conditional validation
• examples/collaboration-document/ - Comments, suggestions, track changes
• examples/phantoms-document/ - Multi-scope annotation clusters

Phase 5: Documentation

• Added terminology glossary to spec/core/03a-anchors-and-references.md
• Created spec/extensions/README.md with compatibility matrix
• Documented extension policies in schemas/README.md
• Added conditional validation docs to forms extension
• Added CI and license badges to root README

Test plan

• All 15 schemas compile successfully
• All 8 example documents validate against schemas
• npm test passes