feat: add source file tracking to rem type

Add a `source` field to `Remind.rem` to track the originating iCalendar
filename (basename without extension). Thread the basename through
`remind_of_event` so each reminder records which file it came from.

bin/remind.ml

@@ -11,6 +11,7 @@ type simple_weekly = {
 (** A simple weekly REM command *)
 
 type rem = {
+  source : string;  (** Source file or identifier for the reminder *)
   original_uuid : string;  (** Original UID from the iCalendar event *)
   summary : string;  (** Summary or title of the reminder *)
   date : Timedesc.Date.t;  (** Date specification (day, month, year) *)
@@ -28,6 +29,7 @@ type rem = {
 
 let empty =
   {
+    source = "";
     original_uuid = "";
     summary = "";
     date = Timedesc.Date.Ymd.make_exn ~year:1970 ~month:1 ~day:1;