ocaml: phase 5.1 grep_count.ml baseline (substring-aware line filter, 3 matches)

Defines a recursive str_contains that walks the haystack with
String.sub to find a needle substring. Real OCaml's String.contains
only accepts a single char, so this baseline implements its own
substring search to stay portable.

  let rec str_contains s sub i =
    if i + sl > nl then false
    else if String.sub s i sl = sub then true
    else str_contains s sub (i + 1)

count_matching splits text on newlines, folds with the predicate.

  'the quick brown fox\nfox runs fast\nthe dog\nfoxes are clever'
   needle = 'fox'
   matches = 3 (lines 1, 2, 4 — 'foxes' contains 'fox')

43 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -16,6 +16,7 @@
   "factorial.ml": 3628800,
   "fraction.ml": 7,
   "frequency.ml": 5,
+  "grep_count.ml": 3,
   "hanoi.ml": 1023,
   "fizzbuzz.ml": 57,
   "list_ops.ml": 30,