The iso8601 flag previously controlled both the calendar grid layout
and the week number format as a coupled boolean. This adds a new
weeks-numbering option (iso/monday/sunday) that explicitly overrides
just the week number calculation method, independent of iso8601 and
locale settings. Existing behaviour is fully preserved when the option
is not set.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
