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.md

@@ -9,4 +9,5 @@ _Generated by `lib/artdag/conformance.sh`_
 | plan | 18 | 0 | 18 |
 | execute | 15 | 0 | 15 |
 | optimize | 18 | 0 | 18 |
-| **Total** | **87** | **0** | **87** |
+| fed | 15 | 0 | 15 |
+| **Total** | **102** | **0** | **102** |