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/scoreboard.json

@@ -7,9 +7,10 @@
     "optimize": {"pass": 22, "fail": 0},
     "fed": {"pass": 15, "fail": 0},
     "cost": {"pass": 13, "fail": 0},
-    "serialize": {"pass": 13, "fail": 0}
+    "serialize": {"pass": 13, "fail": 0},
+    "stats": {"pass": 12, "fail": 0}
   },
-  "total_pass": 132,
+  "total_pass": 144,
   "total_fail": 0,
-  "total": 132
+  "total": 144
 }