mod: Phase 4 — federation (trust, sharing, revocation), 132/132 — roadmap done

Cross-instance reports ingest into the local registry with origin tags; the
engine decides them unchanged. Decision sharing pushes to a mock fed-sx outbox
(mod/fed-send! is the transport seam). Trust is advisory by default: a peer's
decision binds locally only under (mod/trusted? peer :mod), else it lands in the
advisory log unapplied. Revocation composes with the Phase-2 proof model —
fed-revoke-if-invalidated re-runs the engine and undoes moderation only when the
action no longer holds (exoneration flips hide→keep → revoked + origin notified).
+26 fed tests. Full mod-on-sx roadmap complete.

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

lib/mod/conformance.conf

@@ -16,10 +16,12 @@ PRELOADS=(
   lib/mod/lifecycle.sx
   lib/mod/audit.sx
   lib/mod/api.sx
+  lib/mod/fed.sx
 )
 
 SUITES=(
   "decide:lib/mod/tests/decide.sx:(mod-decide-tests-run!)"
   "audit:lib/mod/tests/audit.sx:(mod-audit-tests-run!)"
   "escalation:lib/mod/tests/escalation.sx:(mod-escalation-tests-run!)"
+  "fed:lib/mod/tests/fed.sx:(mod-fed-tests-run!)"
 )