ocaml: phase 5.1 adler32.ml baseline (Adler-32 of 'Wikipedia' = 300286872 = 0x11E60398)

Two running sums modulo 65521:

  a = (1 + sum of bytes)            mod 65521
  b = sum of running 'a' values     mod 65521
  checksum = b * 65536 + a

  let adler32 s =
    let a = ref 1 in
    let b = ref 0 in
    let m = 65521 in
    for i = 0 to String.length s - 1 do
      a := (!a + Char.code s.[i]) mod m;
      b := (!b + !a) mod m
    done;
    !b * 65536 + !a

For 'Wikipedia': 0x11E60398 = 300286872 (the canonical test value).

Tests for-loop accumulating two refs together, modular arithmetic,
and Char.code on s.[i].

96 baseline programs total.

plans/ocaml-on-sx.md

@@ -407,6 +407,11 @@ _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 — adler32.ml baseline (Adler-32 checksum of
+  "Wikipedia" = 300286872 = 0x11E60398). Two running sums modulo
+  65521; final checksum is `b * 65536 + a`. Used by zlib for stream
+  integrity. Tests for-loop accumulating two refs, modular
+  arithmetic, and Char.code on s.[i]. 96 baseline programs total.
 - 2026-05-09 Phase 5.1 — gray_code.ml baseline (4-bit binary
   reflected Gray code, sum 120 + length 16 = 136). Single-formula
   generation: `gray[i] = i lxor (i lsr 1)`. Outputs a permutation of