ocaml: phase 5.1 word_count.ml baseline (10/10 pass)

Uses Map.Make(StrOrd) + List.fold_left to count word frequencies;
exercises the full functor pipeline with a real-world idiom:

  let inc_count m word =
    match StrMap.find_opt word m with
    | None -> StrMap.add word 1 m
    | Some n -> StrMap.add word (n + 1) m
  let count words = List.fold_left inc_count StrMap.empty words

10/10 baseline programs pass.

plans/ocaml-on-sx.md

@@ -386,6 +386,9 @@ the "mother tongue" closure: OCaml → SX → OCaml. This means:
 
 _Newest first._
 
+- 2026-05-08 Phase 5.1 — word_count.ml baseline (10/10 pass). Uses
+  Map.Make(StrOrd) + List.fold_left to count word frequencies; tests
+  the full functor pipeline with a real OCaml idiom.
 - 2026-05-08 Phase 6 — Map/Set extensions: iter/fold/map/filter/
   is_empty + Set.union/inter (+4 tests, 422 total). Functor
   bodies grow naturally — all in OCaml syntax.