events: booking cancellation + seat release + 13 tests

Booking stream carries :booking/:cancel events; live roster is the folded
replay so cancelling frees a seat and capacity reopens. ev/cancel! (retrying
append-expect), no-op on unbooked, cancelled actor may re-book. Capacity count
is folded roster size. 110/110 green.

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

lib/events/tests/booking.sx

@@ -1,4 +1,4 @@
-;; lib/events/tests/booking.sx — capacity-safe transactional booking.
+;; lib/events/tests/booking.sx — capacity-safe transactional booking + cancel.
 
 (define ev-bk-pass 0)
 (define ev-bk-fail 0)
@@ -166,7 +166,69 @@
                 (ev-bk-check!
                   "room is now full"
                   (ev/seats-left b "room" 3)
-                  0)))))))))
+                  0))))))
+      (let
+        ((b (persist/open)))
+        (do
+          (ev/book! b "cx" 2 (quote a))
+          (ev/book! b "cx" 2 (quote c))
+          (ev-bk-check!
+            "occupied to capacity before cancel"
+            (ev/seats-left b "cx" 2)
+            0)
+          (ev-bk-check!
+            "booking when full (pre-cancel) is refused"
+            (get (ev/book! b "cx" 2 (quote d)) :status)
+            :full)
+          (ev-bk-check!
+            "cancel reports cancelled"
+            (get (ev/cancel! b "cx" (quote a)) :status)
+            :cancelled)
+          (ev-bk-check!
+            "cancel removes actor from roster"
+            (ev/roster b "cx")
+            (list (quote c)))
+          (ev-bk-check!
+            "cancel frees a seat"
+            (ev/seats-left b "cx" 2)
+            1)
+          (ev-bk-check!
+            "freed seat is bookable again"
+            (get (ev/book! b "cx" 2 (quote d)) :status)
+            :booked)
+          (ev-bk-check!
+            "roster after rebook is c,d"
+            (ev/roster b "cx")
+            (list (quote c) (quote d)))))
+      (let
+        ((b (persist/open)))
+        (do
+          (ev/book! b "ce" 3 (quote a))
+          (ev-bk-check!
+            "cancelling an unbooked actor is a no-op"
+            (get (ev/cancel! b "ce" (quote z)) :status)
+            :not-booked)
+          (ev-bk-check!
+            "no-op cancel leaves roster intact"
+            (ev/roster b "ce")
+            (list (quote a)))
+          (ev/cancel! b "ce" (quote a))
+          (ev-bk-check!
+            "double cancel is not-booked the second time"
+            (get (ev/cancel! b "ce" (quote a)) :status)
+            :not-booked)
+          (ev-bk-check!
+            "empty roster after cancel"
+            (ev/roster b "ce")
+            (list))
+          (ev-bk-check!
+            "cancelled actor may re-book"
+            (get (ev/book! b "ce" 3 (quote a)) :status)
+            :booked)
+          (ev-bk-check!
+            "re-booked actor back on roster"
+            (ev/roster b "ce")
+            (list (quote a))))))))
 
 (define
   ev-booking-tests-run!