events: fix timezone-aware iCal export (local->UTC stamps) + 6 tests

Bug: tz events store wall-clock LOCAL times but export stamped them with a Z
(UTC) suffix, so a London 18:00 event falsely read as 18:00 UTC. ev-ical-conv
now converts a tz event's DTSTART/UNTIL/EXDATE/RDATE local->UTC before
formatting (London summer 18:00 -> 170000Z; Paris -> 160000Z); non-tz events
unchanged. Caveat: UTC RRULE drifts from wall-clock-stable tz recurrence across
a DST boundary (VTIMEZONE deferred). 366/366 green.

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

lib/events/tests/ical.sx

@@ -266,6 +266,48 @@
               (ev-ic-starts (first events))
               (ev-ic-starts (ev-event (quote a) (ev-dt 2026 6 1 9 0) 30 {:freq :daily :count 3} 1)))))))))
 
+;; ---- timezone-aware export (local wall-clock -> UTC stamps) ----
+(define
+  ev-ic-dtstart
+  (fn (ev) (ev-ic-line (ev/event->ical-lines ev) "DTSTART")))
+
+(define
+  ev-ic-tz-run-all!
+  (fn
+    ()
+    (do
+      (ev-ic-check!
+        "London winter event exports as the same UTC (GMT)"
+        (ev-ic-dtstart (ev-event-tz (quote w) (ev-dt 2026 1 15 18 0) 60 nil 1 ev-tz-london))
+        "DTSTART:20260115T180000Z")
+      (ev-ic-check!
+        "London summer event exports one hour earlier in UTC (BST)"
+        (ev-ic-dtstart (ev-event-tz (quote s) (ev-dt 2026 7 15 18 0) 60 nil 1 ev-tz-london))
+        "DTSTART:20260715T170000Z")
+      (ev-ic-check!
+        "Paris winter (CET +1) exports one hour earlier in UTC"
+        (ev-ic-dtstart (ev-event-tz (quote p) (ev-dt 2026 1 15 18 0) 60 nil 1 ev-tz-paris))
+        "DTSTART:20260115T170000Z")
+      (ev-ic-check!
+        "Paris summer (CEST +2) exports two hours earlier in UTC"
+        (ev-ic-dtstart (ev-event-tz (quote p) (ev-dt 2026 7 15 18 0) 60 nil 1 ev-tz-paris))
+        "DTSTART:20260715T160000Z")
+      (ev-ic-check!
+        "a non-tz event is exported unchanged"
+        (ev-ic-dtstart (ev-event (quote n) (ev-dt 2026 7 15 18 0) 60 nil 1))
+        "DTSTART:20260715T180000Z")
+      ;; EXDATE on a tz event is also converted to UTC
+      (ev-ic-check!
+        "tz event EXDATE is converted to UTC"
+        (ev-ic-line
+          (ev/event->ical-lines
+            (assoc
+              (ev-event-tz (quote s) (ev-dt 2026 7 1 18 0) 60 {:freq :daily :count 3} 1 ev-tz-london)
+              :exdate
+              (list (ev-dt 2026 7 2 18 0))))
+          "EXDATE")
+        "EXDATE:20260702T170000Z"))))
+
 (define
   ev-ical-tests-run!
   (fn
@@ -276,4 +318,5 @@
       (set! ev-ic-failures (list))
       (ev-ic-run-all!)
       (ev-ic-rt-run-all!)
+      (ev-ic-tz-run-all!)
       {:failures ev-ic-failures :total (+ ev-ic-pass ev-ic-fail) :passed ev-ic-pass :failed ev-ic-fail})))