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/optimize-rules.sx

@@ -186,3 +186,28 @@
     (let
       ((o (artdag/opt-reduce dag id)))
       (artdag/opt-last (artdag/dag-order o)))))
+
+; ---- cost-directed: the maude-optimised cone never costs more than the original ----
+; compares the original output cone (dce to id) against the maude-reduced DAG under an
+; injected cost-fn (op params). Monotone per-node costs => optimisation is never a
+; pessimisation: fewer nodes (DCE/dedup) and fused ops (one blur(M+N) for two blurs).
+
+(define
+  artdag/opt-improvement
+  (fn
+    (dag id cost-fn)
+    (let
+      ((orig (artdag/dce dag (list id))) (opt (artdag/opt-reduce dag id)))
+      {:before (artdag/total-work orig cost-fn)
+        :after (artdag/total-work opt cost-fn)
+        :before-path (artdag/critical-path orig cost-fn)
+        :after-path (artdag/critical-path opt cost-fn)
+        :optimized opt})))
+
+(define
+  artdag/opt-cheaper?
+  (fn
+    (dag id cost-fn)
+    (let
+      ((imp (artdag/opt-improvement dag id cost-fn)))
+      (<= (get imp :after) (get imp :before)))))