Add PostgreSQL koans and fix edge cases

[i-am-the-slime]

Summary

• add short PostgreSQL koans covering type-level query behavior and observed runtime contracts
• reject grouped selects whose non-aggregate columns are absent from GROUP BY
• render inserts with no supplied columns as DEFAULT VALUES
• make executeSqlOne reject multi-row results instead of silently returning Nothing
• decode PostgreSQL fractional TIME seconds at millisecond scale

Verification

• spago test — 103/103 tests passed
• compile-fail suite — 73/73 cases passed