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/tests/timezone.sx

@@ -0,0 +1,173 @@
+;; lib/events/tests/timezone.sx — timezones + DST.
+
+(define ev-tz-pass 0)
+(define ev-tz-fail 0)
+(define ev-tz-failures (list))
+
+(define
+  ev-tz-check!
+  (fn
+    (name got expected)
+    (if
+      (= got expected)
+      (set! ev-tz-pass (+ ev-tz-pass 1))
+      (do
+        (set! ev-tz-fail (+ ev-tz-fail 1))
+        (append!
+          ev-tz-failures
+          (str name "\n    expected: " expected "\n    got:      " got))))))
+
+;; Wall-clock (civil + minute-of-day) an occurrence's UTC start maps to in a tz.
+(define
+  ev-tz-local-of
+  (fn
+    (tz utc-dt)
+    (let
+      ((l (ev-tz-utc->local tz utc-dt)))
+      (list (ev-dt->civil l) (ev-dt-tod l)))))
+
+(define
+  ev-tz-run-all!
+  (fn
+    ()
+    (do
+      (let
+        ((nyc (ev-tz-fixed "EST" -300)))
+        (do
+          (ev-tz-check!
+            "fixed zone: utc -> local subtracts 5h"
+            (ev-tz-utc->local
+              nyc
+              (ev-dt 2026 1 1 17 0))
+            (ev-dt 2026 1 1 12 0))
+          (ev-tz-check!
+            "fixed zone: local -> utc adds 5h back"
+            (ev-tz-local->utc
+              nyc
+              (ev-dt 2026 1 1 12 0))
+            (ev-dt 2026 1 1 17 0))
+          (ev-tz-check!
+            "UTC zone is identity"
+            (ev-tz-local->utc
+              ev-tz-utc
+              (ev-dt 2026 6 1 9 0))
+            (ev-dt 2026 6 1 9 0))))
+      (ev-tz-check!
+        "London winter offset is 0 (GMT)"
+        (ev-tz-offset
+          ev-tz-london
+          (ev-dt 2026 1 15 12 0))
+        0)
+      (ev-tz-check!
+        "London summer offset is 60 (BST)"
+        (ev-tz-offset
+          ev-tz-london
+          (ev-dt 2026 7 15 12 0))
+        60)
+      (ev-tz-check!
+        "Paris winter offset is 60 (CET)"
+        (ev-tz-offset
+          ev-tz-paris
+          (ev-dt 2026 1 15 12 0))
+        60)
+      (ev-tz-check!
+        "Paris summer offset is 120 (CEST)"
+        (ev-tz-offset
+          ev-tz-paris
+          (ev-dt 2026 7 15 12 0))
+        120)
+      (ev-tz-check!
+        "DST starts last Sunday of March"
+        (ev-dt->civil
+          (ev-tz-transition
+            2026
+            (ev-tz-rule 3 -1 6 60)))
+        (list 2026 3 29))
+      (ev-tz-check!
+        "DST ends last Sunday of October"
+        (ev-dt->civil
+          (ev-tz-transition
+            2026
+            (ev-tz-rule 10 -1 6 60)))
+        (list 2026 10 25))
+      (ev-tz-check!
+        "09:00 London in winter is 09:00 UTC"
+        (ev-tz-local->utc
+          ev-tz-london
+          (ev-dt 2026 1 15 9 0))
+        (ev-dt 2026 1 15 9 0))
+      (ev-tz-check!
+        "09:00 London in summer is 08:00 UTC"
+        (ev-tz-local->utc
+          ev-tz-london
+          (ev-dt 2026 7 15 9 0))
+        (ev-dt 2026 7 15 8 0))
+      (ev-tz-check!
+        "round trip utc -> local -> utc"
+        (ev-tz-local->utc
+          ev-tz-london
+          (ev-tz-utc->local
+            ev-tz-london
+            (ev-dt 2026 7 15 8 0)))
+        (ev-dt 2026 7 15 8 0))
+      (let
+        ((ev (ev-event-tz (quote standup) (ev-dt 2026 3 27 9 0) 60 {:freq :daily :count 5} 10 ev-tz-london)))
+        (let
+          ((occs (ev-expand ev (ev-date 2026 3 1) (ev-date 2026 4 5))))
+          (do
+            (ev-tz-check!
+              "daily occurrences shift in UTC across the DST boundary"
+              (map (fn (o) (ev-dt-tod (get o :start))) occs)
+              (list 540 540 480 480 480))
+            (ev-tz-check!
+              "but every occurrence stays 09:00 local wall-clock"
+              (map
+                (fn
+                  (o)
+                  (first
+                    (rest (ev-tz-local-of ev-tz-london (get o :start)))))
+                occs)
+              (list 540 540 540 540 540))
+            (ev-tz-check!
+              "occurrence dates are stable in local time"
+              (map
+                (fn
+                  (o)
+                  (ev-civ-d
+                    (first (ev-tz-local-of ev-tz-london (get o :start)))))
+                occs)
+              (list 27 28 29 30 31)))))
+      (let
+        ((wk (ev-event-tz (quote class) (ev-dt 2026 3 23 18 0) 90 {:freq :weekly :byday (list 0)} 5 ev-tz-london)))
+        (let
+          ((occs (ev-expand wk (ev-date 2026 3 1) (ev-date 2026 4 20))))
+          (ev-tz-check!
+            "weekly Monday 18:00 London stays 18:00 local each week"
+            (map
+              (fn
+                (o)
+                (first (rest (ev-tz-local-of ev-tz-london (get o :start)))))
+              occs)
+            (list 1080 1080 1080 1080))))
+      (let
+        ((plain (ev-event (quote p) (ev-dt 2026 3 27 9 0) 60 {:freq :daily :count 3} 1)))
+        (ev-tz-check!
+          "plain event expands naively (no UTC shift)"
+          (map
+            (fn (o) (ev-dt-tod (get o :start)))
+            (ev-expand
+              plain
+              (ev-date 2026 3 1)
+              (ev-date 2026 4 5)))
+          (list 540 540 540))))))
+
+(define
+  ev-timezone-tests-run!
+  (fn
+    ()
+    (do
+      (set! ev-tz-pass 0)
+      (set! ev-tz-fail 0)
+      (set! ev-tz-failures (list))
+      (ev-tz-run-all!)
+      {:failures ev-tz-failures :total (+ ev-tz-pass ev-tz-fail) :passed ev-tz-pass :failed ev-tz-fail})))