events: wire persist-backed booking into api.sx + 10 tests

Durable booking path alongside in-memory: ev/book-occ!, ev/cancel-occ!,
ev/roster-occ, ev/seats-left-occ (capacity from scheduled event); ev/free-p?,
ev/next-free-p, ev/conflicts-p derive availability by replaying persist
booking streams. Reordered conformance preloads. 120/120 green.

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

lib/events/conformance.conf

@@ -19,7 +19,6 @@ PRELOADS=(
   lib/datalog/magic.sx
   lib/events/calendar.sx
   lib/events/availability.sx
-  lib/events/api.sx
   lib/persist/event.sx
   lib/persist/backend.sx
   lib/persist/log.sx
@@ -27,6 +26,7 @@ PRELOADS=(
   lib/persist/concurrency.sx
   lib/persist/api.sx
   lib/events/booking.sx
+  lib/events/api.sx
 )
 
 SUITES=(