events: fix timezone-aware iCal export (local->UTC stamps) + 6 tests

Bug: tz events store wall-clock LOCAL times but export stamped them with a Z
(UTC) suffix, so a London 18:00 event falsely read as 18:00 UTC. ev-ical-conv
now converts a tz event's DTSTART/UNTIL/EXDATE/RDATE local->UTC before
formatting (London summer 18:00 -> 170000Z; Paris -> 160000Z); non-tz events
unchanged. Caveat: UTC RRULE drifts from wall-clock-stable tz recurrence across
a DST boundary (VTIMEZONE deferred). 366/366 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` → **360/360** (Phases 1-4 + 13 ext: …iCalendar export+import round-trip, whole-series booking)
+`bash lib/events/conformance.sh` → **366/366** (Phases 1-4 + 13 ext + tz-aware iCal export fix)
 
 ## Ground rules
 
@@ -88,6 +88,13 @@ lib/events/api.sx ── (events/schedule) (events/book) (events/agenda) ──
 
 ## Progress log
 
+- 2026-06-07 — Fix: timezone-aware iCal export. Bug — tz events store wall-clock
+  LOCAL times, but export stamped them with a `Z` (UTC) suffix, so a London
+  18:00 event falsely read as 18:00 UTC. `ev-ical-conv` now converts a tz
+  event's DTSTART / UNTIL / EXDATE / RDATE local→UTC before formatting (London
+  summer 18:00 → 170000Z; Paris → 160000Z); non-tz events unchanged. Documented
+  caveat: a UTC RRULE drifts from a wall-clock-stable tz recurrence across a DST
+  boundary — full fidelity needs VTIMEZONE (deferred). +6 tests, 366/366 green.
 - 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,