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.

lib/ocaml/baseline/queens.ml

@@ -0,0 +1,35 @@
+(* Baseline: n-queens count for n=6.
+   We count placements of n queens on an n×n board such that no two
+   share a row, column, or diagonal. *)
+
+let safe q queens =
+  let rec go qs offset =
+    match qs with
+    | [] -> true
+    | h :: t ->
+      if h = q then false
+      else if h - q = offset then false
+      else if q - h = offset then false
+      else go t (offset + 1)
+  in
+  go queens 1
+;;
+
+let rec range a b =
+  if a > b then [] else a :: range (a + 1) b
+;;
+
+let rec solve n queens row =
+  if row > n then 1
+  else
+    List.fold_left
+      (fun acc col ->
+        if safe col queens then
+          acc + solve n (col :: queens) (row + 1)
+        else
+          acc)
+      0
+      (range 1 n)
+;;
+
+solve 4 [] 1