Branch 1 of 2 for v2.7 (the .ics topic). Adds the write side of a
hand-rolled RFC 5545 engine (zero deps, stays on kotlinx-datetime):
- domain/ics: IcsText (escape + 75-octet folding), IcsEvent model,
IcsWriter.writeCalendar. Timezone rule: all-day VALUE=DATE, one-off
timed UTC Z, recurring timed TZID-labelled from EVENT_TIMEZONE (no
VTIMEZONE — import resolves TZID against the OS tz db).
- Single-event share from the detail screen (FileProvider + ACTION_SEND).
- Whole-calendar backup of the writable local calendars to a SAF file
(Settings -> Calendars -> Export as .ics), one combined VCALENDAR.
- insertEvent now writes Events.UID_2445; legacy rows fall back to a
stable synthesised UID at export time so a later restore won't dupe.
- EXDATE / RECURRENCE-ID overrides are deliberately skipped this pass
(documented v1 limit; import will skip them too).
Engine + mapper unit-tested. Import (Branch 2, feat/ics-import) ships in
the same v2.7 release; no tag until both land + on-device review.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
