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/scoreboard.json

@@ -4,9 +4,10 @@
     "analyze": {"pass": 16, "fail": 0},
     "plan": {"pass": 18, "fail": 0},
     "execute": {"pass": 15, "fail": 0},
-    "optimize": {"pass": 18, "fail": 0}
+    "optimize": {"pass": 18, "fail": 0},
+    "fed": {"pass": 15, "fail": 0}
   },
-  "total_pass": 87,
+  "total_pass": 102,
   "total_fail": 0,
-  "total": 87
+  "total": 102
 }