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/remind.ml

@@ -50,6 +50,7 @@ type rem = {
   exdate : Icalendar.date_or_datetime list;  (** List of excluded dates for recurring events *)
   overrides : rem list;  (** Single-event REMs generated from non-cancelled RECURRENCE-ID overrides *)
   tz : Timedesc.Time_zone.t option;  (** Timezone of the event's DTSTART, used for UNTIL conversion *)
+  triggers : Timedesc.Span.t list;  (** List of trigger offsets for alarms, in seconds *)
 }
 (** A complete REM command *)
 
@@ -74,6 +75,7 @@ let empty =
     exdate = [];
     overrides = [];
     tz = None;
+    triggers = [];
   }
 
 (* ── buffer primitives ────────────────────────────────────────── *)