events: per-occurrence overrides / reschedule (RECURRENCE-ID) + 6 tests

ev-with-override re-times/re-sizes a single instance of a series (keyed by
original start). ev-expand applies overrides after EXDATE/RDATE: agenda
re-sorts, instance moved out of window is dropped (slot vacated), no-op for a
non-occurring start. assoc for immutable event update. 254/254 green.

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

lib/events/tests/calendar.sx

@@ -530,6 +530,54 @@
           (len (ev-expand plain (ev-date 2026 6 1) (ev-date 2026 7 1)))
           3)))))
 
+;; ---- per-occurrence overrides (reschedule one instance) ----
+(define
+  ev-cal-ov-run-all!
+  (fn
+    ()
+    (let
+      ((base (ev-event (quote standup) (ev-dt 2026 6 1 9 0) 30 {:freq :daily :count 4} 1)))
+      (do
+        ;; reschedule one instance to a new time + duration
+        (let
+          ((moved (ev-with-override base (ev-dt 2026 6 2 9 0) (ev-dt 2026 6 2 14 0) 45)))
+          (let
+            ((occs (ev-expand moved (ev-date 2026 6 1) (ev-date 2026 6 5))))
+            (do
+              (ev-cal-check!
+                "override moves only the targeted instance"
+                (map (fn (o) (ev-dt-tod (get o :start))) occs)
+                (list 540 840 540 540))
+              (ev-cal-check!
+                "override applies the new duration"
+                (map (fn (o) (- (get o :end) (get o :start))) occs)
+                (list 30 45 30 30))
+              (ev-cal-check!
+                "override keeps the series length"
+                (len occs)
+                4))))
+        ;; an instance moved out of the window vacates its slot
+        (let
+          ((movedout (ev-with-override base (ev-dt 2026 6 2 9 0) (ev-dt 2026 6 20 9 0) 30)))
+          (ev-cal-check!
+            "instance moved out of window is dropped, slot vacated"
+            (ev-cal-starts (ev-expand movedout (ev-date 2026 6 1) (ev-date 2026 6 5)))
+            (list (list 2026 6 1) (list 2026 6 3) (list 2026 6 4))))
+        ;; override for a non-existent original start is a no-op
+        (let
+          ((noop (ev-with-override base (ev-dt 2026 6 9 9 0) (ev-dt 2026 6 9 14 0) 45)))
+          (ev-cal-check!
+            "override for a non-occurring start is a no-op"
+            (len (ev-expand noop (ev-date 2026 6 1) (ev-date 2026 6 5)))
+            4))
+        ;; overrides re-sort the agenda when an instance moves earlier
+        (let
+          ((early (ev-with-override base (ev-dt 2026 6 3 9 0) (ev-dt 2026 6 1 7 0) 30)))
+          (ev-cal-check!
+            "an instance moved earlier re-sorts into place"
+            (map (fn (o) (ev-dt-tod (get o :start))) (ev-expand early (ev-date 2026 6 1) (ev-date 2026 6 5)))
+            (list 420 540 540 540)))))))
+
 (define
   ev-calendar-tests-run!
   (fn
@@ -540,4 +588,5 @@
       (set! ev-cal-failures (list))
       (ev-cal-run-all!)
       (ev-cal-ex-run-all!)
+      (ev-cal-ov-run-all!)
       {:failures ev-cal-failures :total (+ ev-cal-pass ev-cal-fail) :passed ev-cal-pass :failed ev-cal-fail})))