ocaml: phase 5.1 partition.ml baseline (stable partition, evens*100 + odds = 3025)

Two ref lists accumulating in reverse, then List.rev'd — preserves
original order:

  let partition pred xs =
    let yes = ref [] in
    let no = ref [] in
    List.iter (fun x ->
      if pred x then yes := x :: !yes
      else no := x :: !no
    ) xs;
    (List.rev !yes, List.rev !no)

  partition (fun x -> x mod 2 = 0) [1..10]
  -> ([2;4;6;8;10], [1;3;5;7;9])

  evens sum * 100 + odds sum = 30 * 100 + 25 = 3025

Tests higher-order predicate, tuple return, and iter-98 let-tuple
destructuring on the call site.

108 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 — partition.ml baseline (stable partition by
+  predicate, 30*100 + 25 = 3025). Two ref lists accumulating in
+  reverse, then List.rev'd — preserves original order. Test:
+  `partition (fun x -> x mod 2 = 0) [1..10]` → ([2;4;6;8;10],
+  [1;3;5;7;9]) → 30*100 + 25 = 3025. Tests higher-order predicate
+  + tuple return + iter-98 let-tuple destructuring. 108 baseline
+  programs total.
 - 2026-05-09 Phase 5.1 — is_prime.ml baseline (count primes ≤ 100 =
   25). Trial division up to √n with early-exit via bool ref. Loop
   2..n calling is_prime, accumulate count. Returns 25 (the canonical