ocaml: phase 5.1 kth_two.ml baseline (8th smallest of two sorted = 8)

Two-pointer merge advancing the smaller-head pointer k times,
without materializing the merged array:

  while !count < k do
    let pick_a =
      if !i = m then false        (* a exhausted, take from b *)
      else if !j = n then true    (* b exhausted, take from a *)
      else a.(!i) <= b.(!j)
    in
    if pick_a then ... else ...;
    count := !count + 1
  done

For a = [1;3;5;7;9;11;13], b = [2;4;6;8;10;12]:
  merged order: 1,2,3,4,5,6,7,8,9,10,11,12,13
  8th element = 8.

Tests nested if/else if/else flowing into a bool, dual-ref
two-pointer loop, separate count counter for k-th constraint.

184 baseline programs total.

plans/ocaml-on-sx.md

@@ -407,6 +407,16 @@ _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.
+- 2026-05-11 Phase 5.1 — kth_two.ml baseline (8th smallest in
+  merged [1;3;5;7;9;11;13] ∪ [2;4;6;8;10;12] = 8). Two-pointer
+  merge that advances the smaller-head pointer k times. Pick rule:
+    pick_a = (j past end) ? true
+           : (i past end) ? false
+           : a[i] <= b[j]
+  Combined order is 1,2,3,4,5,6,7,8,…; 8th element = 8. Tests
+  nested `if/else if/else` flowing into a bool, dual-ref two-
+  pointer loop, separate count counter for the k-th constraint.
+  184 baseline programs total.
 - 2026-05-11 Phase 5.1 — permutations_gen.ml baseline (enumerate all
   24 permutations of [1;2;3;4], count those with first<last = 12).
   Recursive build via fold-pick-recurse: