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.md

@@ -12,4 +12,5 @@ _Generated by `lib/artdag/conformance.sh`_
 | fed | 15 | 0 | 15 |
 | cost | 13 | 0 | 13 |
 | serialize | 13 | 0 | 13 |
-| **Total** | **132** | **0** | **132** |
+| stats | 12 | 0 | 12 |
+| **Total** | **144** | **0** | **144** |