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/api.sx

@@ -249,3 +249,29 @@
   (fn
     (b store actor ws we)
     (> (len (ev/conflicts-p b store actor ws we)) 0)))
+
+;; ---- conflict-checked booking ----
+;; Capacity is per-event, but an attendee should not be double-booked against
+;; THEMSELVES across different events. Would booking `actor` into `occ` overlap
+;; an existing booking of theirs elsewhere? (Derived from persist availability;
+;; an existing booking into `occ` itself is excluded — that's idempotent.)
+(define
+  ev/would-time-conflict?
+  (fn
+    (b store actor occ)
+    (and
+      (not (ev-actor-booked? b (ev-occ-key occ) actor))
+      (not (ev/free-p? b store actor (get occ :start) (get occ :end))))))
+
+;; Book `actor` into `occ` only if it doesn't clash with their other bookings.
+;; Re-booking the same occurrence is idempotent (:already); a clash returns
+;; :time-conflict; otherwise the normal ev/book-occ! result (:booked / :full).
+(define
+  ev/book-checked!
+  (fn
+    (b store actor occ)
+    (cond
+      ((ev-actor-booked? b (ev-occ-key occ) actor) (ev/book-occ! b store actor occ))
+      ((ev/would-time-conflict? b store actor occ)
+        {:status :time-conflict :actor actor :occ-key (ev-occ-key occ)})
+      (else (ev/book-occ! b store actor occ)))))