artdag: Phase 3 Plan — topological batches + parallelism cap + dirty plan + 18 tests

plan.sx schedules a dag into Kahn-wave batches (parallel-safe), splits waves
wider than a cap into sub-batches, and plans incrementally over the dirty
closure only (out-of-set deps treated as satisfied cache hits). plan 18/18,
total 54/54.

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)
+SUITES=(dag analyze plan)
 
 OUT_JSON="lib/artdag/scoreboard.json"
 OUT_MD="lib/artdag/scoreboard.md"
@@ -38,6 +38,7 @@ run_suite() {
 (load "lib/datalog/api.sx")
 (load "lib/artdag/dag.sx")
 (load "lib/artdag/analyze.sx")
+(load "lib/artdag/plan.sx")
 (epoch 2)
 (eval "(define artdag-test-pass 0)")
 (eval "(define artdag-test-fail 0)")