test: comprehensive L3 test suite — point-to-point, end-to-end, aspects

Add 76 integration tests covering:
- Point-to-point: L1 parse safety (16 tests), L2 schema binding (10 tests),
  L3 type compatibility (8 tests), L4 null safety (6 tests)
- End-to-end: full L1-L4 pipeline, cascading failure at each level
- Aspect: invalid SQL, missing tables/columns, type mismatches, typos
- Edge cases: nested/correlated subqueries, all JOIN types, GROUP BY/HAVING,
  UNION, CTEs, 500-column stress test
- Plugin system: dialect names, unsupported language error
- Manifest parsing: minimal, custom levels, schema-file config
- Schema serialisation round-trip

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hyperpolymath