connector-apple-calendar: event-processing parity with google

Port four event-processing improvements from google-calendar to
Apple Calendar:

- Batched saveLinks(): collapse per-event saveLink RPCs into one
  per page via a linksBySource Map. Heavy recurring meetings with
  many exception instances now write as a single link with N
  occurrences instead of N+1 separate saves.
- Order-independent pending-occurrence drain + seen_master
  tracking: end-of-batch drain merges pending occurrences into
  masters present in the batch regardless of arrival order;
  full-pass terminal orphan flush forwards leftover pendings
  whose master was seen this pass (runtime upserts onto existing
  master) and silently drops those whose master never appeared
  (deleted upstream). Replaces the wholesale clear-all-pendings
  at finishSync that silently dropped legitimate cross-page
  cancellations.
- Two-pass quick + full initial sync: quick pass with now->+1y
  front-loads upcoming meetings; full pass walks the 2-year
  backfill range. Both share one sync lock; phase carried in
  SyncState.
- Hash-keyed description notes: key as description-<sha256-8>
  so edits produce new notes (prior versions preserved as
  history) instead of overwriting in place. Apple ICS CREATED
  is per-spec stable across edits, so firstSeenAt isn't needed
  here (unlike Outlook/Google).

Author: KrisBraun