artdag: execution stats / cache analytics + 12 tests

stats.sx reports hit-ratio, cost-weighted work-recomputed/work-saved,
savings-ratio, and exec-summary over an execution record. Verifies cold (0
saved), warm (all saved), and incremental (saved = unchanged, ran = dirty
closure). stats 12/12, total 144/144.

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 cost serialize)
+SUITES=(dag analyze plan execute optimize fed cost serialize stats)
 
 OUT_JSON="lib/artdag/scoreboard.json"
 OUT_MD="lib/artdag/scoreboard.md"
@@ -49,6 +49,7 @@ run_suite() {
 (load "lib/artdag/federation.sx")
 (load "lib/artdag/cost.sx")
 (load "lib/artdag/serialize.sx")
+(load "lib/artdag/stats.sx")
 (epoch 2)
 (eval "(define artdag-test-pass 0)")
 (eval "(define artdag-test-fail 0)")