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,

lib/ocaml/baseline/topo_dfs.ml

@@ -0,0 +1,26 @@
+let n = 6
+let adj = [|
+  [1; 2];
+  [3];
+  [3; 4];
+  [5];
+  [5];
+  []
+|]
+
+let visited = Array.make n false
+let order = ref []
+
+let rec dfs v =
+  if not visited.(v) then begin
+    visited.(v) <- true;
+    List.iter dfs adj.(v);
+    order := v :: !order
+  end
+
+;;
+
+for v = 0 to n - 1 do
+  dfs v
+done;
+List.fold_left (fun acc v -> acc * 10 + v) 0 !order

plans/ocaml-on-sx.md

@@ -407,6 +407,15 @@ _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.
+- 2026-05-10 Phase 5.1 — topo_dfs.ml baseline (DFS-based topo sort
+  on the same 6-node DAG as topo_sort.ml, digit-fingerprint 24135).
+  Cons each node onto the order list AFTER recursing on all its
+  out-edges, giving a reverse topological order. Same graph
+  (0→{1,2}; 1→{3}; 2→{3,4}; 3→{5}; 4→{5}; 5) produces order
+  [0;2;4;1;3;5]. Reduce: 0→0→2→24→241→2413→24135. Complementary
+  to Kahn's (iter 230); tests recursive DFS without an explicit
+  stack and List.fold_left as a horner reduction. 160 baseline
+  programs total.
 - 2026-05-10 Phase 5.1 — radix_sort.ml baseline (LSD radix sort,
   fingerprint a.(0) + a.(7)*1000 = 802002). 8-element array
   [170;45;75;90;802;24;2;66] sorts to [2;24;45;66;75;90;170;802].