events: waitlist + auto-promotion + 21 tests

When full, ev/waitlist! queues actors FIFO (:waitlist/:unwaitlist on the
booking stream; waiting fold independent of the seat fold). ev/waitlist,
ev/waitlist-position, ev/leave-waitlist!. ev/cancel-promote! frees a seat and
auto-promotes the head of the queue to a confirmed booking. Idempotent.
240/240 green.

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

lib/events/booking.sx

@@ -40,6 +40,9 @@
       ((= (first xs) x) i)
       (else (ev-bk-index (rest xs) x (+ i 1))))))
 
+(define ev-bk-append (fn (xs a) (append xs (list a))))
+(define ev-bk-remove (fn (xs a) (filter (fn (x) (not (= x a))) xs)))
+
 ;; ---- per-actor state association list: ((actor state) ...) in join order ----
 
 (define
@@ -269,3 +272,101 @@
   (fn
     (b occ-key capacity)
     (max 0 (- capacity (ev-booking-count b occ-key)))))
+
+;; ---- waitlist ----
+;; When an occurrence is full, actors join a FIFO waitlist (:waitlist /
+;; :unwaitlist events on the same stream). Taking a seat (:booking / :hold)
+;; removes an actor from the queue, so the waitlist fold is independent of the
+;; seat fold. Cancelling/releasing a seat can auto-promote the head of the
+;; queue (a :booking appended for them).
+
+(define
+  ev-fold-waiting
+  (fn
+    (events)
+    (reduce
+      (fn
+        (acc e)
+        (let
+          ((typ (persist/event-type e))
+            (actor (get (persist/event-data e) :actor)))
+          (cond
+            ((= typ :waitlist) (if (ev-bk-member? actor acc) acc (ev-bk-append acc actor)))
+            ((= typ :unwaitlist) (ev-bk-remove acc actor))
+            ((= typ :booking) (ev-bk-remove acc actor))
+            ((= typ :hold) (ev-bk-remove acc actor))
+            (else acc))))
+      (list)
+      events)))
+
+;; The current waitlist queue (FIFO, oldest first).
+(define
+  ev/waitlist
+  (fn (b occ-key) (ev-fold-waiting (persist/read b (ev-booking-stream occ-key)))))
+
+;; 1-based queue position for an actor (0 if not waiting).
+(define
+  ev/waitlist-position
+  (fn (b occ-key actor) (ev-seat-of (ev/waitlist b occ-key) actor)))
+
+;; Book if a seat is free, else join the waitlist. Idempotent: already seated →
+;; :already; already queued → :already-waiting.
+(define
+  ev/waitlist!
+  (fn
+    (b occ-key capacity actor)
+    (let
+      ((seats (ev-booked-actors b occ-key))
+        (waiting (ev/waitlist b occ-key)))
+      (cond
+        ((ev-bk-member? actor seats)
+          {:status :already :seat (ev-seat-of seats actor) :actor actor})
+        ((ev-bk-member? actor waiting)
+          {:status :already-waiting :position (ev-seat-of waiting actor) :actor actor})
+        (else
+          (let
+            ((r (ev/book! b occ-key capacity actor)))
+            (if
+              (= (get r :status) :booked)
+              r
+              (begin
+                (persist/append b (ev-booking-stream occ-key) :waitlist 0 {:actor actor})
+                {:status :waitlisted
+                  :position (+ (len waiting) 1)
+                  :actor actor}))))))))
+
+;; Leave the waitlist. :left or :not-waiting.
+(define
+  ev/leave-waitlist!
+  (fn
+    (b occ-key actor)
+    (if
+      (ev-bk-member? actor (ev/waitlist b occ-key))
+      (begin
+        (persist/append b (ev-booking-stream occ-key) :unwaitlist 0 {:actor actor})
+        {:status :left :actor actor})
+      {:status :not-waiting :actor actor})))
+
+;; Cancel a seat and, if that frees capacity, auto-promote the head of the
+;; waitlist (a confirmed booking). Returns the cancel result plus :promoted
+;; (the actor promoted, or nil).
+(define
+  ev/cancel-promote!
+  (fn
+    (b occ-key capacity actor)
+    (let
+      ((c (ev/cancel! b occ-key actor)))
+      (if
+        (= (get c :status) :cancelled)
+        (let
+          ((waiting (ev/waitlist b occ-key))
+            (seats (ev-booked-actors b occ-key)))
+          (if
+            (and (not (empty? waiting)) (< (len seats) capacity))
+            (let
+              ((promoted (first waiting)))
+              (begin
+                (persist/append b (ev-booking-stream occ-key) :booking 0 {:actor promoted})
+                {:status :cancelled :actor actor :promoted promoted}))
+            {:status :cancelled :actor actor :promoted nil}))
+        c))))