artdag: Phase 7 cost-directed opt-improvement/opt-cheaper? + 5 tests

artdag/opt-improvement compares the original output cone (dce to id) vs the
maude-reduced DAG under an injected cost-fn, returning before/after total-work and
critical-path. opt-cheaper? asserts optimisation never increases cost: the 5-node
chain drops to 2 (work 5->2, path 5->2) and stays cheaper under radius-weighted cost
(5->3); over dedup and untouched DAGs are never pessimised. Consumes cost.sx. Phase 7
base + (later) cost box done. maude-optimize 38/38, total 196/196.

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

lib/artdag/tests/maude-optimize.sx

@@ -268,3 +268,36 @@
   (artdag/node-params
     (artdag/dag-get mo-plain-opt (artdag/opt-last (artdag/dag-order mo-plain-opt))))
   {:radius 3})
+
+; ---- cost-directed: optimisation never increases cost ----
+
+(define
+  mo-rcost
+  (fn (op params) (if (= op "blur") (max 1 (get params :radius)) 1)))
+
+(artdag-test
+  "opt-improvement: const-cost total work drops on fused chain"
+  (let ((imp (artdag/opt-improvement mo-chain5 mo-chain5-id artdag/const-cost)))
+    (list (get imp :before) (get imp :after)))
+  (list 5 2))
+
+(artdag-test
+  "opt-improvement: critical path shrinks under const cost"
+  (let ((imp (artdag/opt-improvement mo-chain5 mo-chain5-id artdag/const-cost)))
+    (< (get imp :after-path) (get imp :before-path)))
+  true)
+
+(artdag-test
+  "opt-cheaper?: fused chain is cheaper under radius-weighted cost"
+  (artdag/opt-cheaper? mo-chain5 mo-chain5-id mo-rcost)
+  true)
+
+(artdag-test
+  "opt-cheaper?: over dedup is cheaper"
+  (artdag/opt-cheaper? mo-dedup mo-dedup-id artdag/const-cost)
+  true)
+
+(artdag-test
+  "opt-cheaper?: untouched DAG keeps equal cost (never a pessimisation)"
+  (artdag/opt-cheaper? mo-plain mo-plain-id artdag/const-cost)
+  true)