events: capacity-safe transactional booking on persist + 24 tests

booking.sx: per-occurrence append-only stream, roster = replay. Booking
decided against an observed (roster, last-seq) snapshot, committed via
persist/append-expect — atomic check+append, no overbooking, no lock.
Explicit last-seat race test: two bookers, one booked, one conflict, roster
capped. Idempotent per actor. 97/97 green.

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

lib/events/conformance.conf

@@ -5,6 +5,8 @@ MODE=dict
 SCOREBOARD_DIR=lib/events
 
 PRELOADS=(
+  spec/stdlib.sx
+  lib/r7rs.sx
   lib/datalog/tokenizer.sx
   lib/datalog/parser.sx
   lib/datalog/unify.sx
@@ -18,10 +20,18 @@ PRELOADS=(
   lib/events/calendar.sx
   lib/events/availability.sx
   lib/events/api.sx
+  lib/persist/event.sx
+  lib/persist/backend.sx
+  lib/persist/log.sx
+  lib/persist/kv.sx
+  lib/persist/concurrency.sx
+  lib/persist/api.sx
+  lib/events/booking.sx
 )
 
 SUITES=(
   "calendar:lib/events/tests/calendar.sx:(ev-calendar-tests-run!)"
   "availability:lib/events/tests/availability.sx:(ev-availability-tests-run!)"
   "api:lib/events/tests/api.sx:(ev-api-tests-run!)"
+  "booking:lib/events/tests/booking.sx:(ev-booking-tests-run!)"
 )