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/ical.sx

@@ -94,11 +94,27 @@
       (str (get e :ord) (ev-ical-wd (get e :wd)))
       (ev-ical-wd e))))
 
+;; A datetime converter for an event: tz-aware events store wall-clock LOCAL
+;; times, so export converts them to UTC (the `Z` stamps are absolute);
+;; non-tz events pass through unchanged.
+;;   CAVEAT: a UTC RRULE recurs at a fixed UTC offset, whereas a tz event's
+;;   expansion stays wall-clock-stable across DST — so for a tz recurrence that
+;;   crosses a DST boundary the exported series drifts by the offset change
+;;   after the boundary. DTSTART and each individual stamp are correct; full
+;;   fidelity would need a VTIMEZONE block (deferred).
+(define
+  ev-ical-conv
+  (fn
+    (event)
+    (let
+      ((tz (get event :tz)))
+      (if (nil? tz) (fn (t) t) (fn (t) (ev-tz-local->utc tz t))))))
+
 ;; ---- RRULE ----
 (define
   ev-ical-rrule
   (fn
-    (rrule)
+    (rrule conv)
     (let
       ((parts (list (str "FREQ=" (ev-ical-freq (get rrule :freq))))))
       (begin
@@ -112,7 +128,7 @@
           (append! parts (str "COUNT=" (get rrule :count))))
         (when
           (not (nil? (get rrule :until)))
-          (append! parts (str "UNTIL=" (ev-ical-dt (get rrule :until)))))
+          (append! parts (str "UNTIL=" (ev-ical-dt (conv (get rrule :until))))))
         (when
           (not (nil? (get rrule :byday)))
           (append!
@@ -139,17 +155,17 @@
   (fn
     (event)
     (let
-      ((lines (list "BEGIN:VEVENT")))
+      ((lines (list "BEGIN:VEVENT")) (conv (ev-ical-conv event)))
       (begin
         (append! lines (str "UID:" (get event :id)))
         (append! lines (str "SUMMARY:" (get event :id)))
-        (append! lines (str "DTSTART:" (ev-ical-dt (get event :dtstart))))
+        (append! lines (str "DTSTART:" (ev-ical-dt (conv (get event :dtstart)))))
         (append!
           lines
           (str "DURATION:" (ev-ical-duration (get event :duration))))
         (when
           (not (nil? (get event :rrule)))
-          (append! lines (ev-ical-rrule (get event :rrule))))
+          (append! lines (ev-ical-rrule (get event :rrule) conv)))
         (when
           (and
             (not (nil? (get event :exdate)))
@@ -158,7 +174,7 @@
             lines
             (str
               "EXDATE:"
-              (ev-ical-join (map ev-ical-dt (get event :exdate)) ","))))
+              (ev-ical-join (map (fn (d) (ev-ical-dt (conv d))) (get event :exdate)) ","))))
         (when
           (and
             (not (nil? (get event :rdate)))
@@ -167,7 +183,7 @@
             lines
             (str
               "RDATE:"
-              (ev-ical-join (map ev-ical-dt (get event :rdate)) ","))))
+              (ev-ical-join (map (fn (d) (ev-ical-dt (conv d))) (get event :rdate)) ","))))
         (append! lines "END:VEVENT")
         lines))))