Comprehensive validator enforcement and adversarial test coverage:

- Relations extension (identity/relations keywords) enforcement
- ucumUnit keyword support and validation
- \ inheritance: nested schemas inherit root \
- \ target validation (reject primitive types)
- Tuple \ existence validation
- Empty \ / URI scheme validation
- Name identifier syntax enforcement
- Enum type checking against declared type
- Recursive \ circular chain detection
- Validation extension gating (warnings without \)
- Units/currency/symbols extension gating

Supersedes #169 and #170.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>