events: southern-hemisphere DST (+8) — 384/384

The :dst zone model assumed northern ordering (dst-start < dst-end, DST =
[start, end)). Southern zones — DST begins ~Oct and ends ~Apr — have
dst-start > dst-end, so the old (>= start AND < end) test was never true and
ev-tz-offset returned the standard offset year-round.

Fix: detect the ordering. start < end → DST is [start, end); start > end →
DST wraps the calendar-year boundary, active when (utc >= start OR utc < end).

Add 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 preceding
Saturday → rule time -480). VTIMEZONE export is already rule-agnostic, so
southern zones round-trip through iCal unchanged (the -480 folds the
from-offset back to the correct local 02:00/03:00 DTSTART).

+8 timezone tests (now 25): summer/winter offsets, both transition dates,
local->utc in both seasons, and a daily expansion crossing the autumn DST-end
that shifts in UTC (1320,1320,1380,1380,1380) while staying 09:00 local.

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` → **376/376** (Phases 1-4 + 13 ext + tz iCal export via TZID + VTIMEZONE)
+`bash lib/events/conformance.sh` → **384/384** (Phases 1-4 + 14 ext + tz iCal export via TZID + VTIMEZONE + southern-hemisphere DST)
 
 ## Ground rules
 
@@ -88,6 +88,19 @@ lib/events/api.sx ── (events/schedule) (events/book) (events/agenda) ──
 
 ## Progress log
 
+- 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