ocaml: phase 5.1 merge_two.ml baseline (merge two sorted lists, length*sum = 441)

Standard two-finger merge with nested match-in-match:

  let rec merge xs ys =
    match xs with
    | [] -> ys
    | x :: xs' ->
      match ys with
      | [] -> xs
      | y :: ys' ->
        if x <= y then x :: merge xs' (y :: ys')
        else y :: merge (x :: xs') ys'

Used as a building block in merge_sort.ml (iter 104) but called out
as its own baseline here.

  merge [1;4;7;10] [2;3;5;8;9]   = [1;2;3;4;5;7;8;9;10]
  length 9, sum 49, product 441.

85 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -48,6 +48,7 @@
   "memo_fib.ml": 75025,
   "mortgage.ml": 1073,
   "merge_sort.ml": 44,
+  "merge_two.ml": 441,
   "module_use.ml": 3,
   "newton_sqrt.ml": 1414,
   "mutable_record.ml": 10,

lib/ocaml/baseline/merge_two.ml

@@ -0,0 +1,14 @@
+let rec merge xs ys =
+  match xs with
+  | [] -> ys
+  | x :: xs' ->
+    match ys with
+    | [] -> xs
+    | y :: ys' ->
+      if x <= y then x :: merge xs' (y :: ys')
+      else y :: merge (x :: xs') ys'
+
+;;
+
+let m = merge [1; 4; 7; 10] [2; 3; 5; 8; 9] in
+List.fold_left (+) 0 m * List.length m

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_two.ml baseline (merge two sorted
+  lists, length*sum = 9*49 = 441). Standard two-finger merge with
+  nested match-in-match. Used as a building block in merge_sort.ml
+  (iter 104) but called out as its own baseline here. Tests two-arg
+  recursion + nested match dispatch + classic comparison-based
+  merge. 85 baseline programs total.
 - 2026-05-09 Phase 5.1 — pow_mod.ml baseline (fast modular
   exponentiation, sum 738639). Recursive exponentiation by squaring:
   even exponent halves and squares, odd exponent multiplies by base