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/expected.json

@@ -10,6 +10,7 @@
   "module_use.ml": 3,
   "mutable_record.ml": 10,
   "option_match.ml": 5,
+  "queens.ml": 2,
   "quicksort.ml": 44,
   "sum_squares.ml": 385,
   "word_count.ml": 3