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/remind.ml

@@ -32,6 +32,7 @@ type rem = {
       (** List of events that are part of the same recurring series: these are only the overrides, not the master event
       *)
   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 *)
 }
 (** A complete REM command *)
 
@@ -49,6 +50,7 @@ let empty =
     daily = None;
     recurring = [];
     exdate = [];
+    overrides = [];
   }
 
 (* ── buffer primitives ────────────────────────────────────────── *)
@@ -221,12 +223,16 @@ let render_yearly rem month day =
 (* ── dispatcher ───────────────────────────────────────────────── *)
 
 let string_of_rem rem =
-  match rem.daily with
-  | Some d -> render_daily rem d
-  | None -> (
-      match rem.weekly with
-      | Some w -> render_weekly rem w
-      | None -> (
-          match rem.yearly with
-          | Some (month, day) -> render_yearly rem month day
-          | None -> render_single rem))
+  let main =
+    match rem.daily with
+    | Some d -> render_daily rem d
+    | None -> (
+        match rem.weekly with
+        | Some w -> render_weekly rem w
+        | None -> (
+            match rem.yearly with
+            | Some (month, day) -> render_yearly rem month day
+            | None -> render_single rem))
+  in
+  let overrides = List.map render_single rem.overrides in
+  String.concat "" (main :: overrides)