YearMonth.from(d).toString() emits the same canonical YYYY-MM string
as the previous String.format("%04d-%02d", …) call but reads as a
single intent-revealing expression. Existing assertions on
"1915-08", "1916-01", … pin the output format unchanged.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>