ocaml: phase 5.1 run_length.ml baseline (RLE, sum-of-counts = 11)

Run-length encoding via tail-recursive 4-arg accumulator:

  let rle xs =
    let rec aux xs cur n acc =
      match xs with
      | [] -> List.rev ((cur, n) :: acc)
      | h :: t ->
        if h = cur then aux t cur (n + 1) acc
        else aux t h 1 ((cur, n) :: acc)
    in
    match xs with
    | [] -> []
    | h :: t -> aux t h 1 []

  rle [1;1;1;2;2;3;3;3;3;1;1] = [(1,3);(2,2);(3,4);(1,2)]
  sum of counts                = 11 (matches input length)

The sum-of-counts test verifies that the encoding preserves total
length — drops or duplicates would diverge.

44 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 — run_length.ml baseline (run-length encoding,
+  sum of counts = 11). RLE encodes [1;1;1;2;2;3;3;3;3;1;1] as
+  [(1,3);(2,2);(3,4);(1,2)]. Sum-of-counts = 11 verifies that the
+  encoding preserves total length. Tail-recursive accumulator with
+  4-arg helper, two-arm dispatch on whether the next element matches
+  the current run head, List.rev to restore order, fold_left with
+  tuple-pattern fun. 44 baseline programs total.
 - 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