- Fix is_null_dynamic to use dynamic.classify for consistency
- Add test: nullable field accepts null value
- Add test: non-nullable field rejects null value
- Add test: nullable field not in properties fails schema validation
- Add test: valid nullable schema passes validation

- Fix double colon bug when validation path is empty
- Add defs.main prefix to data validation errors for clarity
- Add comprehensive tests for error message formats
- Schema validation errors show definition via #defName prefix
- Data validation errors include defs.main in path

- Fix union validator to properly validate local references through
resolution instead of skipping validation
- Add 6 new union tests with full lexicon catalog integration:
* All primitive non-object types validation
* Empty refs in data validation context
* Comprehensive reference matching (local, global main, fragments)
* Schema resolution with constraint validation
* Open vs closed union comparison
* Basic union with full context
- Implement complete params data validation with property type checking
- Add proper data validator to union tests via dispatch_data_validation
- Tests now validate required fields, constraints, and full resolution chain
- All 220 tests passing with full property schema resolution

Test coverage now aligns with reference implementations.