feat(alarms): collect VALARM trigger offsets from iCalendar events

- Add `triggers` field to `Remind.rem` type to store alarm trigger
  offsets as `Timedesc.Span.t list`
- Implement `get_triggers` in `Utils` to extract duration-based triggers
  from audio/display alarms, ignoring email and NONE alarms
- Add `collect_triggers` collector that populates the triggers field and
  logs them to stderr for debugging
- Register `collect_triggers` in the collector pipeline
- Remove leftover debug log for processed filenames in `main.ml`
- Remove stale commented-out RRULE dataset and type documentation from
  `simple_recurrence`

bin/utils.ml

@@ -160,6 +160,28 @@ let get_exdates ev =
 
   List.map (fun d -> `Date d) dates @ List.map (fun dt -> `Datetime dt) datetimes
 
+let get_triggers ev : Timedesc.Span.t list =
+  let alarms = ev.Icalendar.alarms in
+  let triggers =
+    ListLabels.fold_left ~init:[] alarms ~f:(fun acc alarm ->
+        let trigger =
+          match alarm with
+          | `Audio a -> Some a.Icalendar.trigger (* we keep audio triggers *)
+          | `Display d -> Some d.Icalendar.trigger (* we keep display triggers *)
+          | `Email _ -> None (* we ignore email triggers *)
+          | `None _ -> None (* we ignore VAL=NONE triggers *)
+        in
+        match trigger with
+        | Some trigger -> begin
+            let _, trigger_duration_or_datetime = trigger in
+            match trigger_duration_or_datetime with
+            | `Duration dur -> Timedesc.Utils.span_of_ptime_span dur :: acc
+            | `Datetime _ -> acc
+          end
+        | None -> acc)
+  in
+  triggers
+
 let get_rdates ev =
   let uid = get_uid ev in
   let event_props = ev.props in