events: api.sx — public events facade + 14 tests (Phase 1 complete)

Immutable store ({:events :bookings}) over calendar+availability:
ev/schedule, ev/book, ev/agenda, ev/agenda-for, ev/free?, ev/next-free,
ev/conflicts. Availability queries auto-widen expansion by longest event.
73/73 green. Phase 1 done.

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

lib/events/conformance.conf

@@ -17,9 +17,11 @@ PRELOADS=(
   lib/datalog/magic.sx
   lib/events/calendar.sx
   lib/events/availability.sx
+  lib/events/api.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!)"
 )