ocaml: phase 5.1 hailstone.ml baseline (Collatz length from 27 = 111 steps)

Iterative Collatz / hailstone sequence:

  let collatz_length n =
    let m = ref n in
    let count = ref 0 in
    while !m > 1 do
      if !m mod 2 = 0 then m := !m / 2
      else m := 3 * !m + 1;
      count := !count + 1
    done;
    !count

27 is the famous 'long-running' Collatz starter. Reaches a peak of
9232 mid-sequence and takes 111 steps to bottom out at 1.

64 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 — hailstone.ml baseline (Collatz length,
+  starting from 27 → 111 steps to reach 1). Iterative while-loop
+  applies `n / 2` if even, `3n + 1` if odd, counting steps. 27 is
+  the famous "long-running" Collatz starter that produces 111
+  iterations and a peak value of 9232 mid-sequence. 64 baseline
+  programs total.
 - 2026-05-09 Phase 5.1 — twosum.ml baseline (LeetCode #1, hashtable
   one-pass, index-sum 1+3+1 = 5). Walks list with List.iteri,
   checking if `target - x` is already in the hashtable; if yes, the