JSON Schema validator: docs + verify integration (closes #19)

[simbo1905]

Adds brief validator docs + verify integration; CI runs mvn verify; agents prefer mvnd; strict mode available for the suite; ~71% in strict currently.

Changes

• JSON Schema validation module with Draft 2020-12 support
• Integration with official JSON Schema Test Suite
• Maven verify integration with CI
• Documentation and usage examples
• Thread-safe compiled schemas with immutable results
• Error paths and messages for validation failures

Testing

• Unit tests for schema validation features
• Integration tests with official JSON Schema Test Suite
• Default lenient mode skips unsupported tests
• Strict mode available with ~71% pass rate
• CI runs full test suite on push/PR