relations: Phase 4 federation (erel trust-gating, peer_rel/trust, fed-sx mock transport, revocation) + 22 tests

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

lib/relations/conformance.conf

@@ -18,10 +18,12 @@ PRELOADS=(
   lib/relations/engine.sx
   lib/relations/api.sx
   lib/relations/explain.sx
+  lib/relations/federation.sx
 )
 
 SUITES=(
   "direct:lib/relations/tests/direct.sx:(relations-direct-tests-run!)"
   "reach:lib/relations/tests/reach.sx:(relations-reach-tests-run!)"
   "path:lib/relations/tests/path.sx:(relations-path-tests-run!)"
+  "fed:lib/relations/tests/fed.sx:(relations-fed-tests-run!)"
 )