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>

plans/events-on-sx.md

@@ -18,7 +18,7 @@ capacity rules, transactional booking, and a flow-driven notification dispatcher
 
 ## Status (rolling)
 
-`bash lib/events/conformance.sh` → **303/303** (Phases 1-4 + 9 ext: fed f/b, waitlist, EXDATE/RDATE, overrides, booking/reschedule-notify, fed transport, timezones+DST, e2e delivery pipeline)
+`bash lib/events/conformance.sh` → **311/311** (Phases 1-4 + 10 ext: …timezones+DST, e2e delivery pipeline, cross-event conflict-checked booking)
 
 ## Ground rules
 
@@ -88,6 +88,13 @@ lib/events/api.sx ── (events/schedule) (events/book) (events/agenda) ──
 
 ## Progress log
 
+- 2026-06-07 — Cross-event conflict-checked booking (extension). Capacity is
+  per-event, but `ev/book-checked!` also prevents an attendee double-booking
+  THEMSELVES across different events: it consults the actor's persist-derived
+  availability (ev/free-p?) for the occurrence's window and returns
+  :time-conflict on overlap, else the normal ev/book-occ! result. Re-booking
+  the same occurrence is idempotent (:already, not a conflict); other actors are
+  unaffected. `ev/would-time-conflict?` predicate. +8 tests, 311/311 green.
 - 2026-06-07 — End-to-end delivery pipeline (closes the derivation↔delivery
   gap). `ev/deliver-messages` bridges SX notification messages to the Scheme
   notify flow: each (id recipient body) is `serialize`d to s-expression text,