feat(recurring): implement RECURRENCE-ID override handling

- Add `overrides` field to `rem` type to hold single-event REMs from
  non-cancelled overrides
- Add `is_cancelled`, `build_override_rem`, and `collect_overrides` to
  process RECURRENCE-ID override events
- Replace `warn_unhandled_recurring` with `collect_overrides` in the
  collector pipeline
- Fix `separate_master_and_recurrence` partition logic (swapped
  `Left`/`Right`)
- Render override REMs appended to the master REM in `string_of_rem`

bin/utils.ml

@@ -176,8 +176,8 @@ let separate_master_and_recurrence (events : Icalendar.event list) : Icalendar.e
     List.partition_map
       (fun (ev, recur_id_opt) ->
         match recur_id_opt with
-        | None -> Right ev
-        | Some _ -> Left ev)
+        | None -> Left ev (* no RECURRENCE-ID → master *)
+        | Some _ -> Right ev (* has RECURRENCE-ID → override *))
       recur_ids
   in
   match master_and_recurrences with