DatePicker

A calendar that speaks ISO strings, so the date library never reaches your code.

**The value in and out is an ISO string, not a `DateValue`.** Bits builds on
`@internationalized/date`, and that type does not belong in Lauf's API — the
same rule as for the rest of Bits. Askr sends dates as `YYYY-MM-DD` from
`DateTimeToSql`, and that is the form an app should be able to pass straight
in and get straight out. An empty or malformed value gives an empty picker
rather than a blank page.

Bits owns the calendar: arrow keys between days, PageUp and PageDown between
months, a real grid with weekday column headers, and the selected day
announced. It is far more than it looks.

> This is the expensive one: about 200 kB over the floor, because a correct
> calendar is a large amount of code. Worth it on a booking form, not worth it
> on a sign-up page. Tree-shaking means only the pages that import it pay.