artdag: Phase 7 non-vacuous confluence gate regression + 2 tests

Assert mau/confluent? actually discriminates: the Peano-arithmetic variant of the
optimisation laws is flagged non-confluent with named non-joinable pairs, so the green
'opt module is confluent' is real evidence rather than a rubber stamp. maude-optimize
40/40, total 198/198.

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

lib/artdag/scoreboard.json

@@ -10,9 +10,9 @@
     "serialize": {"pass": 13, "fail": 0},
     "stats": {"pass": 12, "fail": 0},
     "fault": {"pass": 14, "fail": 0},
-    "maude-optimize": {"pass": 38, "fail": 0}
+    "maude-optimize": {"pass": 40, "fail": 0}
   },
-  "total_pass": 196,
+  "total_pass": 198,
   "total_fail": 0,
-  "total": 196
+  "total": 198
 }