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>

plans/artdag-on-sx.md

@@ -30,7 +30,7 @@ edges.
 
 ## Status (rolling)
 
-`bash lib/artdag/conformance.sh` → **132/132** (8 suites: dag, analyze, plan, execute, optimize, fed, cost, serialize)
+`bash lib/artdag/conformance.sh` → **144/144** (9 suites: dag, analyze, plan, execute, optimize, fed, cost, serialize, stats)
 
 Base roadmap (Phases 1–6) COMPLETE. Now extending.
 
@@ -138,6 +138,12 @@ lib/artdag/optimize.sx                  lib/artdag/federation.sx
 
 ## Progress log
 
+- **Ext: execution stats / cache analytics** (stats suite 12/12, total 144/144).
+  `lib/artdag/stats.sx` over an exec record: `hit-ratio`, `work-recomputed`/`work-saved`
+  (cost-weighted via the cost model), `savings-ratio`, and `exec-summary`. Cold run =
+  0 hit ratio / all work ran; warm rerun = ratio 1 / all work saved; incremental = saved
+  work counts unchanged nodes, ran work counts the dirty closure.
+
 - **Ext: optimize composition pass** (optimize suite 22/22, total 132/132).
   `artdag/optimize entries outputs fusible?` fuses the entry list then DCEs against
   the output names (sinks survive fusion since they're never absorbed) — fewer nodes,