Wire the previously-dead nuncio_cal::RecurrenceEngine into
CalendarGrpcService::list_events: non-recurring events pass through
unchanged, while recurring masters are expanded into real occurrences
for the requested window via a new store method
(list_recurring_calendar_events, no time filter since a master's own
timestamps can precede the window while its occurrences fall inside
it) and a pure expand_events_for_window merge helper that keeps
recurring rows sourced only from expansion, never double-counting a
master that also overlaps the window in the raw query.
Fix expand_occurrences to stop trusting an empty match as "return the
raw master": that fabricated an out-of-window result whenever a
window fell entirely outside a bounded (UNTIL/COUNT) series, and could
hang scanning an unbounded series for a match that would never come.
skip_while/take_while over the RRuleSet's monotonic iterator bounds
the walk and returns a genuinely empty result instead.
Also apply the calendar_id filter list_events accepted but ignored,
closing the gap between it and the mock backend's own filtering.
Refs #186
James Maes