I changed one line in `src/parse-iso-date.mjs`. The leap-year check is now `year % 4 === 0 && year % 100 !== 0` instead of `year % 4 === 0`, so `1900-02-29` correctly returns `null`.

`npm test` passes (`{"status":"PASS","suite":"parse-iso-date"}`). No other files were changed.