ocaml: phase 5.1 merge_sort.ml baseline (user mergesort, sum=44)

User-implemented mergesort that exercises features added across the
last few iterations:

  let rec split lst = match lst with
    | x :: y :: rest ->
      let (a, b) = split rest in       (* iter 98 let-tuple destruct *)
      (x :: a, y :: b)
    | ...

  let rec merge xs ys = match xs with
    | x :: xs' ->
      match ys with                     (* nested match-in-match *)
      | y :: ys' -> ...

  ...

  List.fold_left (+) 0 (sort [...])     (* iter 89 (op) section *)

Sum of [3;1;4;1;5;9;2;6;5;3;5] = 44 regardless of order, so the
result is also a smoke test of the implementation correctness — if
merge_sort drops or duplicates an element the sum diverges. 26
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 — merge_sort.ml baseline (user-implemented
+  mergesort, sorted sum = 44). Stress-tests `let (a, b) = split rest
+  in (x :: a, y :: b)` (let-tuple destructuring inside a recursive
+  match arm), nested match-in-match for the merge merge step, and
+  the (op) operator section `(+)` as fold accumulator. 26 baseline
+  programs total.
 - 2026-05-09 Phase 4 — top-level `let f (a, b) = body` tuple-param
   decl (+3 tests, 559 total). parse-decl-let (which lives outside
   the ocaml-parse scope and lacks parse-pattern access) uses a