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.

lib/ocaml/baseline/expected.json

@@ -166,6 +166,7 @@
   "shuffle.ml": 55,
   "simpson_int.ml": 10000,
   "stable_unique.ml": 46,
+  "stock_two.ml": 6,
   "subseq_check.ml": 3,
   "tail_factorial.ml": 479001600,
   "tarjan_scc.ml": 4,