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>

plans/artdag-on-sx.md

@@ -30,7 +30,7 @@ edges.
 
 ## Status (rolling)
 
-`bash lib/artdag/conformance.sh` → **36/36** (2 suites: dag, analyze)
+`bash lib/artdag/conformance.sh` → **54/54** (3 suites: dag, analyze, plan)
 
 ## Ground rules
 
@@ -97,10 +97,10 @@ lib/artdag/optimize.sx                  lib/artdag/federation.sx
 
 ## Phase 3 — Plan
 
-- [ ] `lib/artdag/plan.sx` — schedule into topological **batches** (each batch's
+- [x] `lib/artdag/plan.sx` — schedule into topological **batches** (each batch's
   nodes have all deps satisfied → run in parallel); respect a max-parallelism limit
-- [ ] plan over the *dirty* subset only (incremental plan)
-- [ ] `lib/artdag/tests/plan.sx` — batch correctness, parallelism cap, dirty-only plan
+- [x] plan over the *dirty* subset only (incremental plan)
+- [x] `lib/artdag/tests/plan.sx` — batch correctness, parallelism cap, dirty-only plan
 - [ ] (optional/later) miniKanren constraint scheduling — flag, don't block on it
 
 ## Phase 4 — Execute (incremental + memoized)
@@ -136,6 +136,15 @@ lib/artdag/optimize.sx                  lib/artdag/federation.sx
 
 ## Progress log
 
+- **Phase 3 — Plan** (plan suite 18/18, total 54/54). `lib/artdag/plan.sx`:
+  `artdag/plan` schedules a dag into Kahn-wave topological batches — each batch's
+  nodes have all in-scope deps satisfied by earlier batches, so they run in parallel.
+  A `cap` (>0) splits any wave wider than the cap into consecutive sub-batches;
+  `cap<=0` is unlimited. `artdag/plan-dirty` schedules only the dirty closure: deps
+  outside the scheduled set (clean cache hits) count as already satisfied, so a
+  mid-node change yields just `[[changed]…[downstream]]`. Inspection helpers
+  `plan-batches`/`plan-width`/`plan-size`/`plan-flatten`.
+
 - **Phase 2 — Analyze on Datalog** (analyze suite 16/16, total 36/36).
   `lib/artdag/analyze.sx`: `artdag/edge-facts` projects each `(input-id, node-id)`
   pair to an `(edge ...)` fact; `artdag/analyze` builds a `dl-program-data` db with