The 1.27.0 CHANGELOG claims the WAI-ARIA Date Picker Dialog pattern, but
the grid rendered every gridcell as a flat, direct child of role="grid"
with no role="row" grouping per week, and the weekday header cells had
no role at all. Grouped days into 7-day role="row" wrappers (display:
contents keeps them out of the CSS grid track so the day cells still
lay out as direct grid items) and gave weekday header cells
role="columnheader". Empty leading/trailing cells get
role="gridcell" aria-hidden="true" so every row's children stay valid
grid cells.

Documented the remaining known gap while at it: the calendar week
always starts on Sunday regardless of locale.