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/calendar.sx

@@ -503,17 +503,69 @@
                (append base rdate-occs))))
           (ev-dedupe-by-start (ev-sort-occs no-ex)))))))
 
+;; ---- per-occurrence overrides (RFC 5545 RECURRENCE-ID) ----
+;; A single instance of a recurring series can be detached and rescheduled. The
+;; event carries :overrides — a list of (orig-start {:start :duration}) — keyed
+;; by the occurrence's ORIGINAL start. Applied after EXDATE/RDATE. A moved
+;; instance whose new start leaves the window is dropped from this window (the
+;; original slot is vacated); an instance moved INTO the window from outside is
+;; out of scope for a windowed expansion (known stub limitation).
+
+(define
+  ev-assoc-lookup
+  (fn
+    (k pairs)
+    (cond
+      ((empty? pairs) nil)
+      ((= (first (first pairs)) k) (first (rest (first pairs))))
+      (else (ev-assoc-lookup k (rest pairs))))))
+
+(define
+  ev-apply-overrides
+  (fn
+    (id base overrides)
+    (map
+      (fn
+        (o)
+        (let
+          ((ov (ev-assoc-lookup (get o :start) overrides)))
+          (if (nil? ov) o (ev-occ id (get ov :start) (get ov :duration)))))
+      base)))
+
+;; Add an override that reschedules the occurrence originally at `orig-start`
+;; to `new-start` with `new-duration`.
+(define
+  ev-with-override
+  (fn
+    (event orig-start new-start new-duration)
+    (assoc
+      event
+      :overrides
+      (cons
+        (list orig-start {:start new-start :duration new-duration})
+        (ev-or (get event :overrides) (list))))))
+
 ;; Expand an event into occurrence dicts within the window, applying any
-;; EXDATE/RDATE exceptions. This is the public entry point.
+;; EXDATE/RDATE exceptions and per-occurrence overrides. Public entry point.
 (define
   ev-expand
   (fn
     (event win-start win-end)
-    (ev-apply-exceptions
-      event
-      (ev-expand-base event win-start win-end)
-      win-start
-      win-end)))
+    (let
+      ((excepted
+         (ev-apply-exceptions
+           event
+           (ev-expand-base event win-start win-end)
+           win-start
+           win-end))
+        (overrides (ev-or (get event :overrides) (list)))
+        (id (get event :id)))
+      (if
+        (empty? overrides)
+        excepted
+        (filter
+          (fn (o) (and (>= (get o :start) win-start) (<= (get o :start) win-end)))
+          (ev-sort-occs (ev-apply-overrides id excepted overrides)))))))
 
 ;; ---- multi-event expansion (sorted by start) ----