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/scoreboard.json

@@ -5,9 +5,10 @@
     "plan": {"pass": 18, "fail": 0},
     "execute": {"pass": 15, "fail": 0},
     "optimize": {"pass": 18, "fail": 0},
-    "fed": {"pass": 15, "fail": 0}
+    "fed": {"pass": 15, "fail": 0},
+    "cost": {"pass": 13, "fail": 0}
   },
-  "total_pass": 102,
+  "total_pass": 115,
   "total_fail": 0,
-  "total": 102
+  "total": 115
 }