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.

README.md

@@ -75,8 +75,10 @@ source that fails event validation or that Nocal cannot safely round-trip.
 
 Nocal expands and safely round-trips a focused iCalendar recurrence subset:
 daily, weekly, monthly, and yearly rules with interval, count or until, common
-weekly/monthly/yearly selectors, `EXDATE`, floating times, UTC, and system IANA
-`TZID` names. Recurring instances are individually navigable; edit and delete
+weekly/monthly/yearly selectors, compatible DATE or DATE-TIME `RDATE` additions,
+`EXDATE`, floating times, UTC, and system IANA `TZID` names. Occurrences merge
+`DTSTART`, rule expansion, and `RDATE`, remove duplicate starts, and then apply
+`EXDATE`. Recurring instances are individually navigable; edit and delete
 clearly operate on the entire series.
 
 Search expands recurring matches within five years on either side of the
@@ -88,8 +90,9 @@ agenda, appointment focus, and search. Visibility and agenda position are
 session-only presentation state; saves always retain events from hidden
 calendars.
 
-Calendars containing features this version cannot preserve—such as `RDATE`,
-detached recurrence overrides, ordinal `BYDAY`, embedded `VTIMEZONE`
+Calendars containing features this version cannot preserve—such as `RDATE`
+periods or values incompatible with the event's `DTSTART` time basis or `TZID`,
+detached `RECURRENCE-ID` overrides, ordinal `BYDAY`, embedded `VTIMEZONE`
 definitions, alarms, or attendees—remain browsable but read-only. This guard
 prevents a local edit from silently discarding unknown data.