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>

lib/events/tests/ical.sx

@@ -335,6 +335,17 @@
         (do
           (ev-ic-check! "Paris DAYLIGHT goes to +0200 (CEST)" (ev-ic-find vtz "TZOFFSETTO:+0200") "TZOFFSETTO:+0200")
           (ev-ic-check! "Paris STANDARD goes to +0100 (CET)" (ev-ic-find vtz "TZOFFSETTO:+0100") "TZOFFSETTO:+0100")))
+      ;; southern hemisphere exports a valid VTIMEZONE too: reversed offsets,
+      ;; first-Sunday rules, and the -480 rule time folds back to local 02:00/03:00
+      (let
+        ((vtz (ev-ical-vtimezone ev-tz-sydney)))
+        (do
+          (ev-ic-check! "Sydney VTIMEZONE names the zone" (ev-ic-find vtz "TZID") "TZID:Australia/Sydney")
+          (ev-ic-check! "Sydney DAYLIGHT goes to +1100 (AEDT)" (ev-ic-find vtz "TZOFFSETTO:+1100") "TZOFFSETTO:+1100")
+          (ev-ic-check! "Sydney STANDARD goes to +1000 (AEST)" (ev-ic-find vtz "TZOFFSETTO:+1000") "TZOFFSETTO:+1000")
+          (ev-ic-check! "Sydney DAYLIGHT rule is first Sunday of October" (ev-ic-find vtz "RRULE:FREQ=YEARLY;BYMONTH=10") "RRULE:FREQ=YEARLY;BYMONTH=10;BYDAY=1SU")
+          (ev-ic-check! "Sydney STANDARD rule is first Sunday of April" (ev-ic-find vtz "RRULE:FREQ=YEARLY;BYMONTH=4") "RRULE:FREQ=YEARLY;BYMONTH=4;BYDAY=1SU")
+          (ev-ic-check! "Sydney DAYLIGHT begins 02:00 local (AEST std, -480 folded)" (ev-ic-find vtz "DTSTART") "DTSTART:19701004T020000")))
 
       ;; ---- VCALENDAR carries one VTIMEZONE per distinct zone ----
       (let
@@ -371,7 +382,13 @@
         (get
           (ev/ical-lines->event (ev/event->ical-lines (ev-event-tz (quote a) (ev-dt 2026 7 15 18 0) 60 nil 1 ev-tz-london)))
           :dtstart)
-        (ev-dt 2026 7 15 18 0)))))
+        (ev-dt 2026 7 15 18 0))
+      (ev-ic-check!
+        "import parses a southern-zone DTSTART;TZID local time"
+        (get
+          (ev/ical-lines->event (ev/event->ical-lines (ev-event-tz (quote a) (ev-dt 2026 1 15 18 0) 60 nil 1 ev-tz-sydney)))
+          :dtstart)
+        (ev-dt 2026 1 15 18 0)))))
 
 (define
   ev-ical-tests-run!

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 —