artdag: cost-based scheduling — critical path + makespan + speedup + 13 tests

cost.sx: injected cost-fn keeps media costs opaque. critical-path = longest
weighted path (= unlimited-worker makespan); makespan sums each batch's slowest
node (full plan == critical path, serial == total-work); speedup = work/makespan.
cost 13/13, total 115/115.

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

lib/artdag/tests/cost.sx

@@ -0,0 +1,117 @@
+; cost model: critical path, makespan under cap, total work, speedup.
+
+(define
+  cost-CHAIN
+  (artdag/build
+    (list
+      (list "a" "in" (list) {})
+      (list "b" "f" (list "a") {})
+      (list "c" "f" (list "b") {})
+      (list "d" "f" (list "c") {}))))
+
+(define
+  cost-DIA
+  (artdag/build
+    (list
+      (list "a" "in" (list) {})
+      (list "b" "f" (list "a") {})
+      (list "c" "g" (list "a") {})
+      (list "d" "add" (list "b" "c") {} true))))
+
+(define cost-W (artdag/op-cost {:f 2 :add 5}))
+
+; ---- unit cost ----
+
+(artdag-test
+  "critical path: chain is its length"
+  (artdag/critical-path cost-CHAIN artdag/const-cost)
+  4)
+
+(artdag-test
+  "critical path: diamond longest path"
+  (artdag/critical-path cost-DIA artdag/const-cost)
+  3)
+
+(artdag-test
+  "total work: unit cost equals node count"
+  (artdag/total-work cost-DIA artdag/const-cost)
+  4)
+
+(artdag-test
+  "single node critical path is its cost"
+  (artdag/critical-path
+    (artdag/build (list (list "a" "in" (list) {})))
+    artdag/const-cost)
+  1)
+
+; ---- makespan vs cap ----
+
+(artdag-test
+  "full plan makespan equals critical path"
+  (artdag/makespan
+    cost-DIA
+    (artdag/plan cost-DIA 0)
+    artdag/const-cost)
+  (artdag/critical-path cost-DIA artdag/const-cost))
+
+(artdag-test
+  "serial plan makespan equals total work"
+  (artdag/makespan
+    cost-DIA
+    (artdag/plan cost-DIA 1)
+    artdag/const-cost)
+  (artdag/total-work cost-DIA artdag/const-cost))
+
+(artdag-test
+  "capped makespan is never below the critical path"
+  (>=
+    (artdag/makespan
+      cost-DIA
+      (artdag/plan cost-DIA 1)
+      artdag/const-cost)
+    (artdag/critical-path cost-DIA artdag/const-cost))
+  true)
+
+; ---- weighted costs ----
+
+(artdag-test
+  "weighted critical path follows heavy ops"
+  (artdag/critical-path cost-DIA cost-W)
+  8)
+
+(artdag-test
+  "weighted total work sums all node costs"
+  (artdag/total-work cost-DIA cost-W)
+  9)
+
+(artdag-test
+  "op-cost defaults unknown ops to 1"
+  (artdag/total-work
+    (artdag/build (list (list "a" "in" (list) {})))
+    cost-W)
+  1)
+
+(artdag-test
+  "weighted full-plan makespan equals critical path"
+  (artdag/makespan cost-DIA (artdag/plan cost-DIA 0) cost-W)
+  (artdag/critical-path cost-DIA cost-W))
+
+; ---- speedup ----
+
+(artdag-test
+  "serial plan has no speedup"
+  (artdag/speedup
+    cost-DIA
+    (artdag/plan cost-DIA 1)
+    artdag/const-cost)
+  1)
+
+(artdag-test
+  "parallel plan beats serial"
+  (>
+    (artdag/speedup
+      cost-DIA
+      (artdag/plan cost-DIA 0)
+      artdag/const-cost)
+    1)
+  true)