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/conformance.sh

@@ -13,7 +13,7 @@ if [ ! -x "$SX_SERVER" ]; then
   exit 1
 fi
 
-SUITES=(dag analyze plan execute optimize fed)
+SUITES=(dag analyze plan execute optimize fed cost)
 
 OUT_JSON="lib/artdag/scoreboard.json"
 OUT_MD="lib/artdag/scoreboard.md"
@@ -47,6 +47,7 @@ run_suite() {
 (load "lib/artdag/execute.sx")
 (load "lib/artdag/optimize.sx")
 (load "lib/artdag/federation.sx")
+(load "lib/artdag/cost.sx")
 (epoch 2)
 (eval "(define artdag-test-pass 0)")
 (eval "(define artdag-test-fail 0)")