Calendar & date/time

Calendar/DatePicker/DateTimePicker are hard release gates for BeeUI 1.0, with explicit value/timezone/i18n semantics and dedicated i18n/week-start/DST regression coverage.

Content pending

Full date/time documentation is tracked in #221 and will be sourced from docs/decisions/008-datetime-architecture.md and the stable DatePicker/DateTimePicker/Calendar APIs once accepted.