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>

plans/events-on-sx.md

@@ -18,7 +18,7 @@ capacity rules, transactional booking, and a flow-driven notification dispatcher
 
 ## Status (rolling)
 
-`bash lib/events/conformance.sh` → **196/196** (Phases 1-2 + Phase 3: notification flows + reminders)
+`bash lib/events/conformance.sh` → **209/209** (Phases 1-4 complete: calendar, booking, notify, federation)
 
 ## Ground rules
 
@@ -79,11 +79,22 @@ lib/events/api.sx ── (events/schedule) (events/book) (events/agenda) ──
   (request→dispatch→resume, idempotent, bounded retry) is the extraction seam.**
 
 ## Phase 4 — Federation
-- [ ] cross-instance events (peer calendar) — trust-gated stub
-- [ ] tests: federated agenda merge
+- [x] cross-instance events (peer calendar) — trust-gated stub
+- [x] tests: federated agenda merge
+- [ ] federated availability/free-busy across trusted peers
+- [ ] real transport behind `ev/peer-agenda` (fed-sx / signed fetch)
 
 ## Progress log
 
+- 2026-06-07 — **Phase 4: federation (trust-gated stub).** `federation.sx`:
+  a peer publishes a schedule (events store); `ev/federated-agenda` merges the
+  local agenda (origin :local) with every TRUSTED peer's agenda, sorted by
+  start, each occurrence tagged with :origin provenance. Trust is a peer-id set
+  re-checked per merge (revocation is immediate); untrusted peers contribute
+  nothing. `ev/peer`, `ev/trusts?`, `ev/trusted-peers`, `ev/peer-agenda`
+  (expands the peer's recurrence in-window), `ev/from-origin` (filter by
+  source). Real transport slots behind `ev/peer-agenda` unchanged. +13 tests,
+  **209/209 green — all four plan phases implemented.**
 - 2026-06-07 — Reminders + digests from the agenda. `reminders.sx` bridges
   calendar + durable rosters to notify: `ev/occurrence-reminders` (one per
   booked attendee, fires `lead` before start, idempotency key