ocaml: phase 5.1 monotonic.ml baseline (monotonicity check, 4/5 inputs monotonic)

Tracks two bool refs (inc, dec). For each pair of consecutive
elements: if h < prev clear inc, if h > prev clear dec. Returns
inc OR dec at the end:

  let is_monotonic xs =
    match xs with
    | [] -> true
    | [_] -> true
    | _ ->
      let inc = ref true in
      let dec = ref true in
      let rec walk prev rest = ... in
      (match xs with h :: t -> walk h t | [] -> ());
      !inc || !dec

Five test cases:
  [1;2;3;4]    inc only        true
  [4;3;2;1]    dec only        true
  [1;2;1]      neither          false
  [5;5;5]      both (constant)  true
  []            empty            true (vacuous)
  sum = 4

98 baseline programs total.

plans/ocaml-on-sx.md

@@ -407,6 +407,17 @@ _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 — monotonic.ml baseline (monotonicity check,
+  4/5 inputs monotonic). Tracks two bool refs (inc, dec). Each pair
+  of consecutive elements: if `h < prev` clear `inc`, if `h > prev`
+  clear `dec`. Empty list and singleton are vacuously true. Five
+  test cases:
+    [1;2;3;4]   inc only        true
+    [4;3;2;1]   dec only        true
+    [1;2;1]     neither          false
+    [5;5;5]     both (constant)  true
+    []           empty            true (vacuous)
+  Sum = 4. 98 baseline programs total.
 - 2026-05-09 Phase 5.1 — majority_vote.ml baseline (Boyer-Moore
   majority, [3;3;4;2;4;4;2;4;4] → 4). O(n) time / O(1) space:
   candidate-and-count refs; on match increment, on mismatch