ocaml: phase 5.1 unique_set.ml baseline (Set.Make + IntOrd, count = 9)

First baseline that exercises the functor pipeline end to end:

  module IntOrd = struct
    type t = int
    let compare a b = a - b
  end

  module IntSet = Set.Make (IntOrd)

  let unique_count xs =
    let s = List.fold_left (fun s x -> IntSet.add x s) IntSet.empty xs in
    IntSet.cardinal s

Counts unique elements in [3;1;4;1;5;9;2;6;5;3;5;8;9;7;9]:
  {1,2,3,4,5,6,7,8,9} -> 9

The input has 15 elements with 9 unique values. The 'type t = int'
declaration in IntOrd is required by real OCaml; OCaml-on-SX is
dynamic and would accept it without, but we include it for source
fidelity. 27 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 — unique_set.ml baseline (Set.Make + IntOrd
+  functor app, count uniques in [3;1;4;1;5;9;2;6;5;3;5;8;9;7;9] →
+  9). First baseline that exercises the functor pipeline end to
+  end: defines an Ord module with `type t = int` + `compare`, applies
+  Set.Make to it, then folds the input list adding each element to
+  the set and queries `IntSet.cardinal`. 27 baseline programs total.
 - 2026-05-09 Phase 4 — Set.Make / Map.Make functor application
   smoke tests (+3 tests, 572 total). Functors were already wired
   through ocaml-make-functor in eval.sx but had no explicit tests