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.

lib/ocaml/baseline/expected.json

@@ -102,6 +102,7 @@
   "json_pretty.ml": 24,
   "kadane.ml": 6,
   "kmp.ml": 5,
+  "kth_two.ml": 8,
   "knapsack.ml": 36,
   "lambda_calc.ml": 7,
   "lcs.ml": 4,