Merge origin/architecture (events 391/391) — no overlap with W14 gate merge

Co-Authored-By: Claude Fable 5 <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` → **376/376** (Phases 1-4 + 13 ext + tz iCal export via TZID + VTIMEZONE)
+`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 +88,26 @@ 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 —
+  `ev-tz-offset` returned std year-round. Fixed by detecting the ordering: when
+  start < end DST is the interval [start, end); when start > end DST wraps the
+  year boundary (active when `utc ≥ start OR utc < end`). Added predefined
+  `ev-tz-sydney` (AEST +600 / AEDT +660; transitions 02:00 AEST first-Sun-Oct
+  and 03:00 AEDT first-Sun-Apr, both 16:00 UTC the prior Saturday → rule time
+  −480). VTIMEZONE export already rule-agnostic, so southern zones round-trip
+  too (the −480 folds the from-offset back to the correct local 02:00/03:00).
+  +8 tests (timezone 25): summer/winter offsets, both transition dates,
+  local→utc both seasons, and a daily expansion crossing the autumn DST-end that
+  shifts in UTC (1320·1320·1380·1380·1380) yet stays 09:00 local. 384/384 green.
 - 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