artdag: Phase 7 opt-reduce bridges maude normal form back to a runnable DAG + 8 tests

artdag/opt-reduce: encode a DAG cone -> opt-term, mau/creduce against the
optimisation module, decode the normal form back to build-entries and rebuild.
Result-preserving: a 5-node blur;blur;id;bright0 chain collapses to 2 nodes and an
over(I,I) dedup 3->2, both executing identically to the original; non-optimisable
DAGs round-trip their radius faithfully (unary 1+1+1 -> 3). Completes Phase 7's
bridge-back + equivalence boxes. maude-optimize 33/33, total 191/191.

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": 25, "fail": 0}
+    "maude-optimize": {"pass": 33, "fail": 0}
   },
-  "total_pass": 183,
+  "total_pass": 191,
   "total_fail": 0,
-  "total": 183
+  "total": 191
 }