artdag: fault-tolerant execution — confined failure, cache never poisoned + 14 tests

fault.sx run-safe: a node op may return (artdag/fail reason); failure is confined
to that node + downstream dependents while independent branches compute, and failed
results are never cached, so retry after a fix recomputes only the failed closure
and hits the good nodes. fault 14/14, total 158/158.

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

lib/artdag/scoreboard.json

@@ -8,9 +8,10 @@
     "fed": {"pass": 15, "fail": 0},
     "cost": {"pass": 13, "fail": 0},
     "serialize": {"pass": 13, "fail": 0},
-    "stats": {"pass": 12, "fail": 0}
+    "stats": {"pass": 12, "fail": 0},
+    "fault": {"pass": 14, "fail": 0}
   },
-  "total_pass": 144,
+  "total_pass": 158,
   "total_fail": 0,
-  "total": 144
+  "total": 158
 }