ocaml: phase 5.1 max_product3.ml baseline (max product of 3, with negatives -> 300)

Sort, then compare two candidates:
  p1 = product of three largest values
  p2 = product of two smallest (potentially negative) values and the largest

For [-10;-10;1;3;2]:
  sorted    = [-10;-10;1;2;3]
  p1        = 3 * 2 * 1                = 6
  p2        = (-10) * (-10) * 3        = 300
  max       = 300

Tests List.sort + Array.of_list + arr.(n-i) end-walk + candidate-pick
via if-then-else.

104 baseline programs total.

plans/ocaml-on-sx.md

@@ -407,6 +407,12 @@ _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.
+- 2026-05-09 Phase 5.1 — max_product3.ml baseline (max product of
+  three from a list including negatives = 300). Sort, then compare
+  product of three largest vs product of two smallest negatives and
+  one largest. For [-10;-10;1;3;2]: 3*2*1 = 6 vs (-10)*(-10)*3 = 300.
+  Tests List.sort + Array.of_list + arr.(n-i) end-walk + candidate
+  compare. 104 baseline programs total.
 - 2026-05-09 Phase 5.1 — euler9.ml baseline (Project Euler #9, abc
   product for the unique Pythagorean triple with a+b+c=1000 →
   31875000). Naive triple loop times out under contention (10-min