events: federated free/busy across trusted peers + 10 tests

Peers publish busy intervals per actor (iCal free/busy model — privacy-
preserving, not event details). ev/peer-with-busy, ev/peer-busy;
ev/federated-busy unions local availability-db busy + trusted peers' published
busy (sorted); ev/federated-free? answers cross-instance availability,
half-open, trust-gated (untrusted peers ignored). 219/219 green.

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` → **209/209** (Phases 1-4 complete: calendar, booking, notify, federation)
+`bash lib/events/conformance.sh` → **219/219** (Phases 1-4 + ext: federated free/busy)
 
 ## Ground rules
 
@@ -81,11 +81,18 @@ lib/events/api.sx ── (events/schedule) (events/book) (events/agenda) ──
 ## Phase 4 — Federation
 - [x] cross-instance events (peer calendar) — trust-gated stub
 - [x] tests: federated agenda merge
-- [ ] federated availability/free-busy across trusted peers
+- [x] federated availability/free-busy across trusted peers
 - [ ] real transport behind `ev/peer-agenda` (fed-sx / signed fetch)
 
 ## Progress log
 
+- 2026-06-07 — Federated free/busy (extension). Peers publish BUSY intervals
+  per actor (iCal free/busy model — privacy-preserving, not event details).
+  `ev/peer-with-busy`, `ev/peer-busy`; `ev/federated-busy` unions local
+  availability-db busy + trusted peers' published busy (sorted);
+  `ev/federated-free?` answers "is X free in [qs,qe)?" across instances,
+  half-open, trust-gated (untrusted peers' busy ignored; revocation immediate).
+  +10 tests, 219/219 green.
 - 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