feat: support safe recurrence additions

Parse, persist, expand, and round-trip compatible RDATE values while preserving whole-series TUI behavior and conservative rewrite guards.

Verified: normal, warning-as-error, and ASan/UBSan builds pass all 9 suites; storage failure paths preserve calendar and backup bytes; live PTY restores terminal state.

include/nocal/domain/event.hpp

@@ -39,6 +39,7 @@ struct Event {
     TimeBasis time_basis{TimeBasis::utc};
     std::string time_zone;
     std::optional<RecurrenceRule> recurrence;
+    std::vector<TimePoint> recurrence_additions;
     std::vector<TimePoint> recurrence_exceptions;
 };
 
@@ -55,4 +56,9 @@ struct Calendar {
     return event.end >= event.start;
 }
 
+[[nodiscard]] constexpr bool is_recurring(const Event& event) noexcept
+{
+    return event.recurrence.has_value() || !event.recurrence_additions.empty();
+}
+
 } // namespace nocal