events: iCal coverage for southern-hemisphere VTIMEZONE (+7) — 391/391

The previous commit asserted southern zones round-trip through iCal unchanged
but verified it only by reasoning. Close that gap with explicit tests:

- A Sydney VTIMEZONE export block: TZID:Australia/Sydney, DAYLIGHT->+1100
  (AEDT) / STANDARD->+1000 (AEST), first-Sunday rules (BYMONTH=10/4 BYDAY=1SU),
  and DAYLIGHT DTSTART:19701004T020000 — confirming the -480 rule time folds
  the from-offset back to the correct local 02:00 AEST transition.
- A southern-zone DTSTART;TZID export -> import round-trip preserving :dtstart.

+7 ical tests (now 63).

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` → **384/384** (Phases 1-4 + 14 ext + tz iCal export via TZID + VTIMEZONE + southern-hemisphere DST)
+`bash lib/events/conformance.sh` → **391/391** (Phases 1-4 + 14 ext + tz iCal export via TZID + VTIMEZONE + southern-hemisphere DST incl. iCal round-trip)
 
 ## Ground rules
 
@@ -88,6 +88,13 @@ lib/events/api.sx ── (events/schedule) (events/book) (events/agenda) ──
 
 ## Progress log
 
+- 2026-06-10 — Harden southern-hemisphere DST: explicit iCal coverage for the
+  previous commit's unverified claim that "southern zones round-trip through
+  iCal unchanged". Added a Sydney VTIMEZONE export block (TZID:Australia/Sydney,
+  DAYLIGHT→+1100/STANDARD→+1000, first-Sunday rules BYMONTH=10/4 BYDAY=1SU, and
+  DAYLIGHT DTSTART:19701004T020000 — proving the −480 rule time folds back to
+  local 02:00 AEST) and a southern-zone DTSTART;TZID export→import round-trip.
+  +7 tests (ical 63). 391/391 green.
 - 2026-06-10 — Southern-hemisphere DST. The `:dst` zone model assumed northern
   ordering (dst-start < dst-end, DST = [start, end)); southern zones (DST begins
   ~Oct, ends ~Apr) have dst-start > dst-end and so silently never entered DST —