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/conformance.sh

@@ -13,7 +13,7 @@ if [ ! -x "$SX_SERVER" ]; then
   exit 1
 fi
 
-SUITES=(dag analyze plan)
+SUITES=(dag analyze plan execute)
 
 OUT_JSON="lib/artdag/scoreboard.json"
 OUT_MD="lib/artdag/scoreboard.md"
@@ -36,9 +36,15 @@ run_suite() {
 (load "lib/datalog/strata.sx")
 (load "lib/datalog/eval.sx")
 (load "lib/datalog/api.sx")
+(load "lib/persist/event.sx")
+(load "lib/persist/backend.sx")
+(load "lib/persist/log.sx")
+(load "lib/persist/kv.sx")
+(load "lib/persist/api.sx")
 (load "lib/artdag/dag.sx")
 (load "lib/artdag/analyze.sx")
 (load "lib/artdag/plan.sx")
+(load "lib/artdag/execute.sx")
 (epoch 2)
 (eval "(define artdag-test-pass 0)")
 (eval "(define artdag-test-fail 0)")