Validate natural joins and add comprehensive test suite

- Natural joins now error when relations have no common columns,
  preventing silent degradation to cross products. The database is
  queried at execution time to resolve column names.
- Added vitest with 70 tests covering all RA operators: selection,
  projection, rename, cross product, natural join (with validation),
  theta join, outer joins, semi-joins, anti-join, division, distinct,
  sort, grouping, set operations, composition, precedence, and errors.

https://claude.ai/code/session_01TJyw8nESra9cc5RpVUpmt6

Author: claude