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/scoreboard.json

@@ -1,9 +1,10 @@
 {
   "suites": {
     "dag": {"pass": 20, "fail": 0},
-    "analyze": {"pass": 16, "fail": 0}
+    "analyze": {"pass": 16, "fail": 0},
+    "plan": {"pass": 18, "fail": 0}
   },
-  "total_pass": 36,
+  "total_pass": 54,
   "total_fail": 0,
-  "total": 36
+  "total": 54
 }