artdag: Phase 4 Execute — content-addressed memo + incremental recompute + 15 tests

execute.sx folds a plan, runs each node via an injected runner (perform in
prod, op-table in tests), and memoizes results in a lib/persist kv backend
keyed by content-id. Incremental recompute falls out of content addressing:
a leaf change reassigns ids across its dirty closure, so re-running hits the
unchanged nodes and recomputes only the closure (cold 5 -> rerun 0 -> change 3).
Cross-dag subgraph sharing verified. execute 15/15, total 69/69.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/artdag/scoreboard.json

@@ -2,9 +2,10 @@
   "suites": {
     "dag": {"pass": 20, "fail": 0},
     "analyze": {"pass": 16, "fail": 0},
-    "plan": {"pass": 18, "fail": 0}
+    "plan": {"pass": 18, "fail": 0},
+    "execute": {"pass": 15, "fail": 0}
   },
-  "total_pass": 54,
+  "total_pass": 69,
   "total_fail": 0,
-  "total": 54
+  "total": 69
 }