ocaml: phase 5.1 word_freq.ml baseline (Map.Make on String, distinct = 8)

First baseline using Map.Make on a string-keyed map:

  module StringOrd = struct
    type t = string
    let compare = String.compare
  end
  module SMap = Map.Make (StringOrd)

  let count_words text =
    let words = String.split_on_char ' ' text in
    List.fold_left (fun m w ->
      let n = match SMap.find_opt w m with
              | Some n -> n
              | None -> 0
      in
      SMap.add w (n + 1) m
    ) SMap.empty words

For 'the quick brown fox jumps over the lazy dog' ('the' appears
twice), SMap.cardinal -> 8.

Complements bag.ml (Hashtbl-based) and unique_set.ml (Set.Make)
with a sorted Map view of the same kind of counting problem. 35
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 — word_freq.ml baseline (Map.Make on String,
+  count distinct words → 8). Defines a StringOrd module + applies
+  Map.Make to it. Folds the input through SMap.find_opt + SMap.add to
+  count each word, then reports SMap.cardinal. "the quick brown fox
+  jumps over the lazy dog" — "the" appears twice, so 8 distinct
+  words. First baseline using Map.Make on a string-keyed map. 35
+  baseline programs total.
 - 2026-05-09 Phase 6 — Either module + Hashtbl.copy (+4 tests, 602
   total). Either: left, right, is_left, is_right, find_left,
   find_right, map_left, map_right, fold, equal, compare. Constructors