ocaml: phase 5.1 hamming.ml baseline (Hamming distance, 3+2-1 = 4)

Counts position-wise differences between two strings of equal
length; returns -1 sentinel for length mismatch:

  let hamming s t =
    if String.length s <> String.length t then -1
    else
      let d = ref 0 in
      for i = 0 to String.length s - 1 do
        if s.[i] <> t.[i] then d := !d + 1
      done;
      !d

Three test cases:
  'karolin'  vs 'kathrin'    3   (positions 2,3,4)
  '1011101'  vs '1001001'    2   (positions 2,4)
  'abc'      vs 'abcd'      -1   (length mismatch)
  sum                        4

91 baseline programs total.

plans/ocaml-on-sx.md

@@ -407,6 +407,13 @@ _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 — hamming.ml baseline (Hamming distance,
+  3 + 2 + (-1) = 4). Counts position-wise differences in equal-length
+  strings; returns -1 sentinel for length mismatch.
+    karolin vs kathrin   3 (positions 2,3,4)
+    1011101 vs 1001001   2 (positions 2,4)
+    abc     vs abcd     -1 (length mismatch)
+  Sum = 4. 91 baseline programs total.
 - 2026-05-09 Phase 5.1 — xor_cipher.ml baseline (XOR roll-key
   encryption, round-trip → 601). For each character, XOR with the
   corresponding key char (key cycled via `i mod kn`). Encrypts