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>

lib/events/timezone.sx

@@ -13,8 +13,11 @@
 ;;   :fixed  — a constant offset.
 ;;   :dst    — std/dst offsets + two transition rules. Transitions are given in
 ;;             UTC (EU zones all switch at 01:00 UTC), so the offset at any UTC
-;;             instant is a direct range check; no recursion. Northern-hemisphere
-;;             ordering (dst-start < dst-end within a year) is assumed.
+;;             instant is a direct range check; no recursion. Both hemispheres
+;;             are supported: northern zones have dst-start < dst-end (DST is the
+;;             interval [start, end)); southern zones have dst-start > dst-end
+;;             (DST wraps the year boundary), detected by comparing the two
+;;             transitions — see ev-tz-offset.
 ;;
 ;; Requires calendar.sx (ev-dt, ev-days-from-civil, ev-civil-from-days,
 ;; ev-civ-y, ev-floor-div, ev-resolve-nth-weekday).
@@ -58,10 +61,20 @@
           (let
             ((start (ev-tz-transition year (get tz :dst-start)))
               (end (ev-tz-transition year (get tz :dst-end))))
+            ;; Northern hemisphere: dst-start < dst-end, DST is the closed-open
+            ;; interval [start, end). Southern hemisphere: dst-start > dst-end
+            ;; (DST begins in spring ~Oct and ends ~Apr), so within a calendar
+            ;; year DST wraps the boundary — active OUTSIDE [end, start).
             (if
-              (and (>= utc-dt start) (< utc-dt end))
-              (get tz :dst-offset)
-              (get tz :std-offset)))))
+              (< start end)
+              (if
+                (and (>= utc-dt start) (< utc-dt end))
+                (get tz :dst-offset)
+                (get tz :std-offset))
+              (if
+                (or (>= utc-dt start) (< utc-dt end))
+                (get tz :dst-offset)
+                (get tz :std-offset))))))
       (else 0))))
 
 ;; UTC instant -> local wall-clock.
@@ -98,6 +111,19 @@
     120
     (ev-tz-rule 3 -1 6 60)
     (ev-tz-rule 10 -1 6 60)))
+;; Southern hemisphere: AEST +600 (std, winter), AEDT +660 (dst, summer). DST
+;; begins 02:00 AEST first Sunday October and ends 03:00 AEDT first Sunday April
+;; — both 16:00 UTC the preceding Saturday, i.e. -480 minutes from the Sunday in
+;; the rule (the model adds rule :time to the resolved weekday's UTC midnight).
+;; dst-start (Oct) > dst-end (Apr), so ev-tz-offset takes the wrap-the-year path.
+(define
+  ev-tz-sydney
+  (ev-tz-dst
+    "Australia/Sydney"
+    600
+    660
+    (ev-tz-rule 10 1 6 -480)
+    (ev-tz-rule 4 1 6 -480)))
 
 ;; ---- tz-aware event expansion ----