ocaml: phase 5.1 rolling_hash.ml baseline (Rabin-Karp, 6 "abc" matches)

Polynomial rolling hash mod 1000003 with base 257:
  - precompute base^(m-1)
  - slide window updating hash in O(1) per step
  - verify hash match with O(m) memcmp to skip false positives

  rolling_match "abcabcabcabcabcabc" "abc" = 6

Six non-overlapping copies of "abc" at positions 0,3,6,9,12,15.

Tests `for _ = 0 to m - 2 do … done` unused loop variable
(uses underscore wildcard pattern), Char.code arithmetic, mod
arithmetic with intermediate negative subtractions, complex nested
if/begin branching with inner break-via-flag.

151 baseline programs total.

plans/ocaml-on-sx.md

@@ -407,6 +407,15 @@ _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.
+- 2026-05-10 Phase 5.1 — rolling_hash.ml baseline (Rabin-Karp
+  rolling hash for substring matching, count "abc" in
+  "abcabcabcabcabcabc" = 6). Polynomial hash mod 1000003 with
+  base 257; precompute base^(m-1), then slide window updating
+  hash in O(1) per step. Verify hash matches with O(m) memcmp to
+  avoid false positives. Tests `for _ = 0 to m - 2 do … done`
+  unused loop variable, char-code arithmetic, mod under negative
+  intermediate, complex nested if/begin/end branching.
+  151 baseline programs total.
 - 2026-05-10 Phase 5.1 — huffman.ml baseline (Huffman tree weighted
   path length on letters {(5,a) (9,b) (12,c) (13,d) (16,e) (45,f)}
   = 224). Builds optimal prefix code by repeatedly merging the two