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/tests/maude-optimize.sx

@@ -301,3 +301,45 @@
   "opt-cheaper?: untouched DAG keeps equal cost (never a pessimisation)"
   (artdag/opt-cheaper? mo-plain mo-plain-id artdag/const-cost)
   true)
+
+; ---- the confluence gate is meaningful, not vacuous ----
+; the Peano-arithmetic variant of the same laws is KNOWN non-confluent (M+0 sticks,
+; (A+B)+C vs A+(B+C) don't join). Assert the checker actually catches it, so the
+; green "opt module is confluent" above is real evidence, not a checker that passes
+; everything.
+
+(define
+  mo-peano-module
+  (mau/parse-module
+    (str
+      "fmod ARTDAGPEANO is\n"
+      "  sorts Img Num .\n"
+      "  op src : -> Img .\n"
+      "  op 0 : -> Num .\n"
+      "  op s_ : Num -> Num .\n"
+      "  op _+_ : Num Num -> Num .\n"
+      "  op blur : Img Num -> Img .\n"
+      "  op bright : Img Num -> Img .\n"
+      "  op id : Img -> Img .\n"
+      "  op over : Img Img -> Img [comm] .\n"
+      "  vars I J : Img .\n"
+      "  vars M N : Num .\n"
+      "  eq 0 + N = N .\n"
+      "  eq s M + N = s (M + N) .\n"
+      "  eq id(I) = I .\n"
+      "  eq blur(I, 0) = I .\n"
+      "  eq bright(I, 0) = I .\n"
+      "  eq blur(blur(I, M), N) = blur(I, M + N) .\n"
+      "  eq bright(bright(I, M), N) = bright(I, M + N) .\n"
+      "  eq over(I, I) = I .\n"
+      "endfm")))
+
+(artdag-test
+  "confluence gate is real: Peano variant is flagged non-confluent"
+  (mau/confluent? mo-peano-module)
+  false)
+
+(artdag-test
+  "confluence gate is real: Peano variant names its non-joinable pairs"
+  (> (len (mau/non-joinable-pairs mo-peano-module)) 0)
+  true)