ocaml: phase 5.1 topo_dfs.ml baseline (DFS topo sort, fingerprint 24135)

DFS topological sort — recurse on out-edges first, prepend after:

  let rec dfs v =
    if not visited.(v) then begin
      visited.(v) <- true;
      List.iter dfs adj.(v);
      order := v :: !order
    end

Same 6-node DAG as iter 230's Kahn's-algorithm baseline:
  0 -> {1, 2}
  1 -> {3}
  2 -> {3, 4}
  3 -> {5}
  4 -> {5}
  5

DFS order: [0; 2; 4; 1; 3; 5]
Horner fold: 0->0->2->24->241->2413->24135.

Complementary to topo_sort.ml (Kahn's BFS); tests recursive DFS
with no explicit stack + List.fold_left as a horner reduction.

160 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -143,6 +143,7 @@
   "tarjan_scc.ml": 4,
   "subset_sum.ml": 8,
   "tic_tac_toe.ml": 1,
+  "topo_dfs.ml": 24135,
   "topo_sort.ml": 6,
   "word_freq.ml": 8,
   "xor_cipher.ml": 601,