events: iCalendar import + occurrence-exact round-trip + 19 tests

ical.sx parses VEVENT/VCALENDAR text back into events (ev/ical-lines->event,
ev/parse-vcalendar): DTSTART/DURATION/RRULE (ordinal BYDAY, BYMONTHDAY, UNTIL/
COUNT/INTERVAL) + EXDATE/RDATE. Round-trip is occurrence-exact — export->import
expands to the identical occurrence set. Completes bidirectional interop.
360/360 green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

plans/events-on-sx.md

@@ -18,7 +18,7 @@ capacity rules, transactional booking, and a flow-driven notification dispatcher
 
 ## Status (rolling)
 
-`bash lib/events/conformance.sh` → **341/341** (Phases 1-4 + 12 ext: …conflict-checked booking, iCalendar export, whole-series booking)
+`bash lib/events/conformance.sh` → **360/360** (Phases 1-4 + 13 ext: …iCalendar export+import round-trip, whole-series booking)
 
 ## Ground rules
 
@@ -88,6 +88,13 @@ lib/events/api.sx ── (events/schedule) (events/book) (events/agenda) ──
 
 ## Progress log
 
+- 2026-06-07 — iCalendar import / round-trip (extension). `ical.sx` now parses
+  VEVENT/VCALENDAR text back into events (`ev/ical-lines->event`,
+  `ev/parse-vcalendar`): DTSTART/DURATION/RRULE (incl. ordinal BYDAY, BYMONTHDAY,
+  UNTIL/COUNT/INTERVAL) and EXDATE/RDATE. Round-trip is occurrence-exact —
+  export→import expands to the identical occurrence set (tested across one-off /
+  daily-count / weekly+exdate+rdate / monthly-ordinal / bymonthday). Completes
+  bidirectional interop. +19 tests, 360/360 green.
 - 2026-06-07 — Whole-series booking (extension). `ev/book-series!` /
   `ev/cancel-series!` apply a booking/cancel to every occurrence of one event
   in a window (e.g. RSVP the whole weekly class), returning per-occurrence