events: cross-event conflict-checked booking + 8 tests

ev/book-checked! prevents an attendee double-booking themselves across
different events: consults their persist-derived availability (ev/free-p?) for
the occurrence window, returns :time-conflict on overlap else the normal
ev/book-occ! result. Re-booking the same occurrence stays idempotent
(:already); other actors unaffected. ev/would-time-conflict? predicate.
311/311 green.

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

lib/events/tests/api.sx

@@ -259,6 +259,66 @@
                             20))
                         true))))))))))))
 
+;; ---- conflict-checked booking ----
+(define
+  ev-api-cf-run-all!
+  (fn
+    ()
+    (let
+      ((b (persist/open))
+        (store
+          (ev/schedule
+            (ev/schedule
+              (ev/schedule (ev/empty) (quote a) (ev-dt 2026 6 1 9 0) 60 nil 10)
+              (quote bb)
+              (ev-dt 2026 6 1 9 30)
+              60
+              nil
+              10)
+            (quote c)
+            (ev-dt 2026 6 1 11 0)
+            60
+            nil
+            10)))
+      (let
+        ((oa (ev-occ (quote a) (ev-dt 2026 6 1 9 0) 60))
+          (ob (ev-occ (quote bb) (ev-dt 2026 6 1 9 30) 60))
+          (oc (ev-occ (quote c) (ev-dt 2026 6 1 11 0) 60)))
+        (do
+          (ev-api-check!
+            "first checked booking succeeds"
+            (get (ev/book-checked! b store (quote nia) oa) :status)
+            :booked)
+          (ev-api-check!
+            "overlapping different-event booking is a time conflict"
+            (get (ev/book-checked! b store (quote nia) ob) :status)
+            :time-conflict)
+          (ev-api-check!
+            "the clashing booking did not land on the roster"
+            (ev/roster-occ b ob)
+            (list))
+          (ev-api-check!
+            "a non-overlapping booking is allowed"
+            (get (ev/book-checked! b store (quote nia) oc) :status)
+            :booked)
+          (ev-api-check!
+            "re-booking the same occurrence is idempotent, not a conflict"
+            (get (ev/book-checked! b store (quote nia) oa) :status)
+            :already)
+          ;; a different actor is unaffected by nia's bookings
+          (ev-api-check!
+            "another actor may take the overlapping slot"
+            (get (ev/book-checked! b store (quote ola) ob) :status)
+            :booked)
+          (ev-api-check!
+            "would-time-conflict? predicate agrees"
+            (ev/would-time-conflict? b store (quote nia) ob)
+            true)
+          (ev-api-check!
+            "would-time-conflict? false for a free slot"
+            (ev/would-time-conflict? b store (quote zed) ob)
+            false))))))
+
 (define
   ev-api-tests-run!
   (fn
@@ -268,4 +328,5 @@
       (set! ev-api-fail 0)
       (set! ev-api-failures (list))
       (ev-api-run-all!)
+      (ev-api-cf-run-all!)
       {:failures ev-api-failures :total (+ ev-api-pass ev-api-fail) :passed ev-api-pass :failed ev-api-fail})))