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/conformance.sh

@@ -13,7 +13,7 @@ if [ ! -x "$SX_SERVER" ]; then
   exit 1
 fi
 
-SUITES=(dag analyze plan execute optimize fed cost serialize stats)
+SUITES=(dag analyze plan execute optimize fed cost serialize stats fault)
 
 OUT_JSON="lib/artdag/scoreboard.json"
 OUT_MD="lib/artdag/scoreboard.md"
@@ -50,6 +50,7 @@ run_suite() {
 (load "lib/artdag/cost.sx")
 (load "lib/artdag/serialize.sx")
 (load "lib/artdag/stats.sx")
+(load "lib/artdag/fault.sx")
 (epoch 2)
 (eval "(define artdag-test-pass 0)")
 (eval "(define artdag-test-fail 0)")