artdag: Phase 6 federation — shared content-addressed cache + trust + invalidation + 15 tests

federation.sx: instance = {cache, prov cid->peer}. fed-export/import share results
by global content-id (trusted import -> pure cache hit, the L2-registry analog);
trust gating rejects untrusted peers; fed-pull uses an injected fetch transport;
fed-invalidate drops a peer's provenanced results (peer-scoped, leaves local
results). fed 15/15, total 102/102. All 6 phases complete.

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

lib/artdag/conformance.sh

@@ -13,7 +13,7 @@ if [ ! -x "$SX_SERVER" ]; then
   exit 1
 fi
 
-SUITES=(dag analyze plan execute optimize)
+SUITES=(dag analyze plan execute optimize fed)
 
 OUT_JSON="lib/artdag/scoreboard.json"
 OUT_MD="lib/artdag/scoreboard.md"
@@ -46,6 +46,7 @@ run_suite() {
 (load "lib/artdag/plan.sx")
 (load "lib/artdag/execute.sx")
 (load "lib/artdag/optimize.sx")
+(load "lib/artdag/federation.sx")
 (epoch 2)
 (eval "(define artdag-test-pass 0)")
 (eval "(define artdag-test-fail 0)")