events: paid-ticket contract (commerce) over holds + 31 tests (Phase 2 done)

ticket.sx: checkout-request (events->commerce) + payment-result
(commerce->events) wire shapes — commerce imports the contract. ev/request-
ticket! holds a seat + emits a checkout request; ev/settle-payment! confirms
on :paid, releases on failure/expiry. Idempotent; late paid for a vanished
hold -> :paid-but-no-hold (refund signal). 175/175 green.

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

lib/events/conformance.conf

@@ -26,6 +26,7 @@ PRELOADS=(
   lib/persist/concurrency.sx
   lib/persist/api.sx
   lib/events/booking.sx
+  lib/events/ticket.sx
   lib/events/api.sx
 )
 
@@ -34,4 +35,5 @@ SUITES=(
   "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!)"
+  "ticket:lib/events/tests/ticket.sx:(ev-ticket-tests-run!)"
 )