ocaml: phase 5.1 stock_two.ml baseline (best of 2 transactions = 6)

Two-pass partition DP for max profit with at most 2 transactions:

  left[i]  = max single-trans profit in prices[0..i]
              (forward scan tracking running min)
  right[i] = max single-trans profit in prices[i..n-1]
              (backward scan tracking running max)
  answer   = max over i of (left[i] + right[i])

For [3; 3; 5; 0; 0; 3; 1; 4]:
  optimal partition i = 2:
    left[2]  = sell@5 after buy@3            = 2
    right[2] = sell@4 after buy@0 in [2..7]  = 4
                                       total = 6

Tests parallel forward + backward passes on parallel DP arrays,
mixed ref + array state, for downto + for ascending scans on
the same data.

190 baseline programs total.

plans/ocaml-on-sx.md

@@ -407,6 +407,16 @@ _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.
+- 2026-05-11 Phase 5.1 — stock_two.ml baseline (max stock profit
+  with at most 2 transactions on [3;3;5;0;0;3;1;4] = 6). Two-pass
+  DP: left[i] = max single-transaction profit in prices[0..i]
+  (forward, tracking running min), right[i] = max single-transaction
+  profit in prices[i..n-1] (backward, tracking running max). Final
+  answer = max over i of left[i]+right[i], partitioning at the
+  best split. Optimal: buy@3, sell@5 (profit 2); buy@0, sell@4
+  (profit 4); total = 6. Tests parallel forward + backward passes
+  on parallel DP arrays, mixed ref+array state, downto + ascending
+  scans on same data. 190 baseline programs total.
 - 2026-05-11 Phase 5.1 — house_robber.ml baseline (linear-DP
   max non-adjacent-sum on [2;7;9;3;1;5;8;6] = 22). dp[i] =
   max(dp[i-2]+houses[i], dp[i-1]). Optimal pick {7, 9, 5, 8} but