events: Phase 4 federation — trust-gated peer agenda merge + 13 tests

federation.sx: a peer publishes a schedule; ev/federated-agenda merges local
(origin :local) with trusted peers' agendas, sorted by start, tagged with
:origin provenance. Trust is a peer-id set re-checked per merge; untrusted
peers contribute nothing. Real transport slots behind ev/peer-agenda.
209/209 green — all four plan phases implemented.

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

lib/events/conformance.conf

@@ -41,6 +41,7 @@ PRELOADS=(
   lib/events/notify.sx
   lib/events/api.sx
   lib/events/reminders.sx
+  lib/events/federation.sx
 )
 
 SUITES=(
@@ -51,4 +52,5 @@ SUITES=(
   "ticket:lib/events/tests/ticket.sx:(ev-ticket-tests-run!)"
   "notify:lib/events/tests/notify.sx:(ev-notify-tests-run!)"
   "reminders:lib/events/tests/reminders.sx:(ev-reminders-tests-run!)"
+  "federation:lib/events/tests/federation.sx:(ev-federation-tests-run!)"
 )