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/scoreboard.json

@@ -1,8 +1,8 @@
 {
   "lang": "events",
-  "total_passed": 196,
+  "total_passed": 209,
   "total_failed": 0,
-  "total": 196,
+  "total": 209,
   "suites": [
     {"name":"calendar","passed":37,"failed":0,"total":37},
     {"name":"availability","passed":22,"failed":0,"total":22},
@@ -10,7 +10,8 @@
     {"name":"booking","passed":61,"failed":0,"total":61},
     {"name":"ticket","passed":31,"failed":0,"total":31},
     {"name":"notify","passed":7,"failed":0,"total":7},
-    {"name":"reminders","passed":14,"failed":0,"total":14}
+    {"name":"reminders","passed":14,"failed":0,"total":14},
+    {"name":"federation","passed":13,"failed":0,"total":13}
   ],
-  "generated": "2026-06-07T04:34:36+00:00"
+  "generated": "2026-06-07T04:58:42+00:00"
 }