events: availability.sx — free/busy + conflict detection on Datalog + 16 tests

occurrence/booking EDB; rules busy/conflict (canonical pair, half-open
overlap)/busy_in. API ev-busy, ev-conflicts, ev-has-conflict?, ev-free?
(transient qwindow). Integrates with calendar expansion. 53/53 green.

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

lib/events/conformance.conf

@@ -16,8 +16,10 @@ PRELOADS=(
   lib/datalog/api.sx
   lib/datalog/magic.sx
   lib/events/calendar.sx
+  lib/events/availability.sx
 )
 
 SUITES=(
   "calendar:lib/events/tests/calendar.sx:(ev-calendar-tests-run!)"
+  "availability:lib/events/tests/availability.sx:(ev-availability-tests-run!)"
 )