feat: RECURRENCE-ID parsing, serialization, and round-trip support

Add recurrence_id field to Event domain model. Parse and serialize
RECURRENCE-ID from/to iCalendar files. Override events (with
RECURRENCE-ID but no RRULE) are safe to rewrite. Save validation
rejects events with both RECURRENCE-ID and RRULE.

include/nocal/domain/event.hpp

@@ -50,6 +50,7 @@ struct Event {
     std::optional<RecurrenceRule> recurrence;
     std::vector<TimePoint> recurrence_additions;
     std::vector<TimePoint> recurrence_exceptions;
+    std::optional<TimePoint> recurrence_id;
 };
 
 struct Calendar {
@@ -70,4 +71,11 @@ struct Calendar {
     return event.recurrence.has_value() || !event.recurrence_additions.empty();
 }
 
+// True when this event is a detached override of a recurring series
+// (has a RECURRENCE-ID but is not itself a recurring series).
+[[nodiscard]] constexpr bool is_override(const Event& event) noexcept
+{
+    return event.recurrence_id.has_value();
+}
+
 } // namespace nocal