rsvp_announcement_event: bridge recipe

Bridge recipe that adds optional event attachment to announcements. Fields to add to announcement: - field_attached_event (entity reference to eventseries, optional) - field_calendar (entity reference — which calendar to post to) - Inline entity form for creating a new eventseries from within the announcement form Must preserve D7 recurrence features: - Daily / weekly / monthly / yearly patterns - Exclusion and inclusion dates - Occurrence limits Requires: rsvp_announcement (core) + rsvp_recurring_events already applied.