events: VTIMEZONE iCal export — full DST-correct tz recurrence + 16 tests

A tz event now exports DTSTART;TZID=<name>:<local> (EXDATE/RDATE likewise;
UNTIL stays UTC per RFC), and the VCALENDAR emits a VTIMEZONE per distinct zone
with DAYLIGHT/STANDARD sub-components generated from the zone's transition rules
(offsets + FREQ=YEARLY;BYMONTH;BYDAY) — London/Paris blocks match real-world
definitions. Clients recur at fixed wall-clock time, DST-correct (prior caveat
gone). Importer tolerates ;TZID= params. 376/376 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` → **366/366** (Phases 1-4 + 13 ext + tz-aware iCal export fix)
+`bash lib/events/conformance.sh` → **376/376** (Phases 1-4 + 13 ext + tz iCal export via TZID + VTIMEZONE)
 
 ## Ground rules
 
@@ -88,6 +88,15 @@ lib/events/api.sx ── (events/schedule) (events/book) (events/agenda) ──
 
 ## Progress log
 
+- 2026-06-07 — VTIMEZONE iCal export (supersedes the UTC-Z tz fix — full DST
+  fidelity). A tz event now exports DTSTART;TZID=<name>:<local> (+ EXDATE/RDATE
+  in the same TZID-local form; UNTIL stays UTC per RFC), and the VCALENDAR emits
+  a VTIMEZONE per distinct zone with DAYLIGHT/STANDARD sub-components generated
+  from the zone's transition rules (offsets + FREQ=YEARLY;BYMONTH;BYDAY) — the
+  London/Paris blocks match real-world definitions exactly. So a client recurs
+  the event at a fixed WALL-CLOCK time, DST-correct (the prior caveat is gone).
+  `ev-ical-vtimezone`, `ev-ical-offset`, distinct-zone collection; importer now
+  tolerates the ;TZID= parameter. +16 tests (ical 56), 376/376 green.
 - 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