core-time: add LocalDate.isoWeekNumber() (ISO-8601 week-of-year)

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>

CHANGELOG.md

@@ -18,6 +18,8 @@ are no binary releases; "version" tracks the shared `version` in the root build
   - `Instant.formatDate()` / `formatTime()` / `formatDateTime()` — locale- and
     zone-aware display formatting over `kotlin.time.Instant`.
   - `TimeBridge` — `Long` epoch-millis ↔ `Instant`, for content-provider I/O.
+  - `LocalDate.isoWeekNumber()` — ISO-8601 week-of-week-based-year, the
+    calendar-week number for week/month grids (one scheme, shared).
 - **`core-crash`** (Android library) — privacy-respecting, on-device crash
   capture and reporting. Writes an allowlist-only report (app/Android/device
   version, locale, time, stack trace — nothing else) to private storage and