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.

plans/ocaml-on-sx.md

@@ -407,6 +407,13 @@ _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 — grep_count.ml baseline (substring-aware
+  line filter, 3 lines match). Defines a recursive `str_contains`
+  that walks the haystack with `String.sub` slices to find a needle
+  substring (real OCaml's `String.contains` only takes a char).
+  Splits text on `'\n'` then folds with the contains predicate. Test
+  text has 4 lines, 3 contain 'fox' (incl 'foxes'). 43 baseline
+  programs total.
 - 2026-05-09 Phase 5.1 — pretty_table.ml baseline (Buffer + Printf
   width specifiers, total length 64). Builds a 4-row scoreboard via
   Buffer + `Printf.sprintf "%-10s %4d\n"`. Each row is exactly 16