Centralises the calendar-week-number computation — ISO-8601 week-of-week-based-year (Monday-start, week 1 holds the first Thursday) — that Calendula previously inlined identically in its Week and Month grid headers. Names the scheme choice (there are several incompatible "week number" definitions) and covers the year-boundary rollover cases with a unit test. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>