feat(event-predicates): warn on unhandled recurrence overrides and multiple masters

- Add `warn_unhandled_recurring` collector that emits a warning when
  `RECURRENCE-ID` overrides are present but not yet handled, emitting
  the master event as-is
- Register `warn_unhandled_recurring` in `all_collectors`
- Warn when more than one master event (no `RECURRENCE-ID`) is found for
  a UID in `separate_master_and_recurrence`, using only the first

bin/utils.ml

@@ -182,4 +182,8 @@ let separate_master_and_recurrence (events : Icalendar.event list) : Icalendar.e
   in
   match master_and_recurrences with
   | [], _ -> failwith "No master event found"
-  | master :: _, recurrences -> (master, recurrences)
+  | [ master ], recurrences -> (master, recurrences)
+  | master :: rest, recurrences ->
+      Printf.eprintf "Warning: %d extra master events (no RECURRENCE-ID) for UID: %s — only first used\n"
+        (List.length rest) (get_uid master);
+      (master, recurrences)