events: timezone + DST support + 17 tests

timezone.sx: wall-clock LOCAL <-> absolute UTC. :fixed + :dst zones (std/dst
offsets + UTC transition rules, EU-style, no IANA DB) computed via calendar
helpers. ev-event-tz authors in local time; ev-expand expands tz events in
LOCAL time then converts each occurrence to UTC, so a 09:00 weekly meeting
stays 09:00 across a DST change (UTC instant shifts). Predefined utc/london/
paris. Plain events unaffected. 295/295 green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/events/calendar.sx

@@ -545,10 +545,9 @@
         (list orig-start {:start new-start :duration new-duration})
         (ev-or (get event :overrides) (list))))))
 
-;; Expand an event into occurrence dicts within the window, applying any
-;; EXDATE/RDATE exceptions and per-occurrence overrides. Public entry point.
+;; Naive (single time-domain) expansion: RRULE + EXDATE/RDATE + overrides.
 (define
-  ev-expand
+  ev-expand-naive
   (fn
     (event win-start win-end)
     (let
@@ -567,6 +566,20 @@
           (fn (o) (and (>= (get o :start) win-start) (<= (get o :start) win-end)))
           (ev-sort-occs (ev-apply-overrides id excepted overrides)))))))
 
+;; Public entry point. A tz-aware event (`:tz` set) expands in local wall-clock
+;; time and converts each occurrence to UTC (ev-expand-tz, timezone.sx); a plain
+;; event expands naively in a single time domain. The window is UTC either way.
+(define
+  ev-expand
+  (fn
+    (event win-start win-end)
+    (let
+      ((tz (get event :tz)))
+      (if
+        (nil? tz)
+        (ev-expand-naive event win-start win-end)
+        (ev-expand-tz event tz win-start win-end)))))
+
 ;; ---- multi-event expansion (sorted by start) ----
 
 ;; Insertion of one occurrence into a start-ascending list.