Recurrences of 28 and 30 days do not align to months, so the domain needs
day-level maths for the first time. It gets it the way months.ts does: an
integer ordinal, no Date anywhere. A Date would make 2026-03-29 a different
instant per timezone while being the same day everywhere, and a 28-day step
must not gain or lose a day at a DST boundary.
Hinnant's days_from_civil, which shifts the year to start in March so the leap
day is the last day rather than an insertion, removing every special case.
No isDayGranular export: nothing in the plan consumes it, later tasks branch
on a recurrence kind and import validation uses a regex directly.
