ocaml: phase 5.1 queens.ml baseline (15/15 pass)

4-queens via recursive backtracking + List.fold_left. Returns 2 (the
two solutions of 4-queens). Per-program timeout in run.sh bumped to
240s — the tree-walking interpreter is slow on heavy recursion but
correct.

The substrate handles full backtracking + safe-check recursion +
list-driven candidate enumeration end-to-end.

plans/ocaml-on-sx.md

@@ -407,6 +407,12 @@ _Newest first._
   binary search tree (`type 'a tree = Leaf | Node of 'a * 'a tree *
   'a tree`) with insert + in-order traversal. Tests parametric ADT,
   recursive match, List.append, List.fold_left.
+- 2026-05-08 Phase 5.1 — queens.ml baseline (15/15 pass). 4-queens
+  count via recursive backtracking with `List.fold_left`. Returns 2
+  (the two solutions of 4-queens). Per-program timeout in run.sh
+  bumped to 240s — tree-walking interpreter is slow on heavy recursion
+  but correct. The substrate handles full backtracking + safe-check
+  recursion + list-driven candidate enumeration end-to-end.
 - 2026-05-08 Phase 5.1 — mutable_record.ml baseline (14/14 pass).
   Counter-style record with two mutable fields, bump function uses
   `r.f <- v` to mutate. End-to-end validates type decl + record