ocaml: phase 5.1 euler40_small.ml baseline (Champernowne digit product = 15)

Build the Champernowne string '12345678910111213...' until at
least 1500 chars; product of digits at positions 1, 10, 100, 1000
is 1 * 1 * 5 * 3 = 15.

Initial implementation timed out: 'String.length (Buffer.contents
buf) < 1500' rebuilt the full string each iteration (O(n^2) in our
spec-level evaluator). Fixed by tracking length separately from
the Buffer:

  let len = ref 0 in
  while !len < 1500 do
    let s = string_of_int !i in
    Buffer.add_string buf s;
    len := !len + String.length s;
    i := !i + 1
  done

Real PE40 uses positions up to 10^6 (answer 210); 1000 keeps under
budget while exercising the same string-build + char-pick pattern.

122 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -36,6 +36,7 @@
   "euler29_small.ml": 15,
   "euler30_cube.ml": 1301,
   "euler34_small.ml": 145,
+  "euler40_small.ml": 15,
   "euler3.ml": 29,
   "euler4_small.ml": 9009,
   "euler5.ml": 232792560,