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/timezone.sx

@@ -0,0 +1,131 @@
+;; lib/events/timezone.sx — timezones + DST for the calendar.
+;;
+;; Datetimes in calendar.sx are naive epoch-minutes (wall clock). A timezone
+;; maps between wall-clock LOCAL time and absolute UTC. An event is authored in
+;; local time + a tz; recurrence is expanded in local time (so a "09:00 weekly"
+;; meeting stays 09:00 across a DST change), then each occurrence is converted
+;; to UTC for storage/comparison.
+;;
+;; Offset convention: offset = local - utc (minutes). London summer (BST) = +60.
+;; UTC = local - offset; local = utc + offset.
+;;
+;; Two kinds of zone, no IANA database:
+;;   :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.
+;;
+;; Requires calendar.sx (ev-dt, ev-days-from-civil, ev-civil-from-days,
+;; ev-civ-y, ev-floor-div, ev-resolve-nth-weekday).
+
+;; A DST transition rule: the ord-th weekday `wd` (0=Mon..6=Sun) of `month`, at
+;; `time` minutes-of-day UTC. EU: last Sunday (ord -1, wd 6) at 01:00 UTC.
+(define ev-tz-rule (fn (month ord wd time) {:ord ord :wd wd :month month :time time}))
+
+(define ev-tz-fixed (fn (name offset) {:name name :offset offset :kind :fixed}))
+
+(define ev-tz-dst (fn (name std dst start-rule end-rule) {:name name :kind :dst :dst-end end-rule :dst-start start-rule :std-offset std :dst-offset dst}))
+
+;; Standard (winter) offset — the initial guess when inverting local -> utc.
+(define
+  ev-tz-std-offset
+  (fn
+    (tz)
+    (if (= (get tz :kind) :fixed) (get tz :offset) (get tz :std-offset))))
+
+;; The UTC instant (epoch-minutes) of a transition rule in a given year.
+(define
+  ev-tz-transition
+  (fn
+    (year rule)
+    (let
+      ((day (ev-resolve-nth-weekday year (get rule :month) (get rule :ord) (get rule :wd))))
+      (+
+        (* (ev-days-from-civil year (get rule :month) day) 1440)
+        (get rule :time)))))
+
+;; The offset (minutes) in effect at a UTC instant.
+(define
+  ev-tz-offset
+  (fn
+    (tz utc-dt)
+    (cond
+      ((= (get tz :kind) :fixed) (get tz :offset))
+      ((= (get tz :kind) :dst)
+        (let
+          ((year (ev-civ-y (ev-civil-from-days (ev-floor-div utc-dt 1440)))))
+          (let
+            ((start (ev-tz-transition year (get tz :dst-start)))
+              (end (ev-tz-transition year (get tz :dst-end))))
+            (if
+              (and (>= utc-dt start) (< utc-dt end))
+              (get tz :dst-offset)
+              (get tz :std-offset)))))
+      (else 0))))
+
+;; UTC instant -> local wall-clock.
+(define
+  ev-tz-utc->local
+  (fn (tz utc-dt) (+ utc-dt (ev-tz-offset tz utc-dt))))
+
+;; Local wall-clock -> UTC instant. The offset depends on the instant, so we
+;; guess with the standard offset and refine once (correct except within the
+;; one-hour DST gap/overlap, where it resolves to the pre-transition offset).
+(define
+  ev-tz-local->utc
+  (fn
+    (tz local-dt)
+    (let
+      ((utc1 (- local-dt (ev-tz-offset tz (- local-dt (ev-tz-std-offset tz))))))
+      (- local-dt (ev-tz-offset tz utc1)))))
+
+;; ---- predefined zones ----
+(define ev-tz-utc (ev-tz-fixed "UTC" 0))
+(define
+  ev-tz-london
+  (ev-tz-dst
+    "Europe/London"
+    0
+    60
+    (ev-tz-rule 3 -1 6 60)
+    (ev-tz-rule 10 -1 6 60)))
+(define
+  ev-tz-paris
+  (ev-tz-dst
+    "Europe/Paris"
+    60
+    120
+    (ev-tz-rule 3 -1 6 60)
+    (ev-tz-rule 10 -1 6 60)))
+
+;; ---- tz-aware event expansion ----
+
+;; An event authored in local time + a tz. dtstart-local / rrule / exceptions
+;; are all wall-clock in `tz`; expansion converts each occurrence to UTC.
+(define
+  ev-event-tz
+  (fn (id dtstart-local duration rrule capacity tz) {:id id :duration duration :dtstart dtstart-local :rrule rrule :capacity capacity :tz tz}))
+
+;; Expand a tz-aware event over a UTC window. Local recurrence is expanded over
+;; a window widened by a day each side (to catch occurrences whose UTC lands in
+;; range), converted to UTC, then filtered to [win-start, win-end].
+(define
+  ev-expand-tz
+  (fn
+    (event tz win-start win-end)
+    (let
+      ((local-ws (- (ev-tz-utc->local tz win-start) 1440))
+        (local-we (+ (ev-tz-utc->local tz win-end) 1440)))
+      (let
+        ((local-occs (ev-expand-naive event local-ws local-we)))
+        (let
+          ((utc-occs (map (fn (o) (let ((u (ev-tz-local->utc tz (get o :start))) (dur (- (get o :end) (get o :start)))) {:id (get o :id) :start u :end (+ u dur)})) local-occs)))
+          (ev-sort-occs
+            (filter
+              (fn
+                (o)
+                (and
+                  (>= (get o :start) win-start)
+                  (<= (get o :start) win-end)))
+              utc-occs)))))))