ocaml: phase 5.1 run_decode.ml baseline (RLE decode, expansion sum = 21)

Inverse of run_length.ml from iteration 130. Takes a list of
(value, count) tuples and expands:

  let rec rle_decode pairs =
    match pairs with
    | [] -> []
    | (x, n) :: rest ->
      let rec rep k = if k = 0 then [] else x :: rep (k - 1) in
      rep n @ rle_decode rest

  rle_decode [(1,3); (2,2); (3,4); (1,2)]
  = [1;1;1; 2;2; 3;3;3;3; 1;1]
  sum = 3 + 4 + 12 + 2 = 21.

Tests tuple-cons pattern, inner-let recursion, list concat (@), and
the 'List.fold_left (+) 0' invariant on encoding round-trips.

81 baseline programs total.

plans/ocaml-on-sx.md

@@ -407,6 +407,12 @@ _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_decode.ml baseline (RLE decode, sum of
+  expansion = 21). Inverse of run_length.ml: takes a list of
+  `(value, count)` tuples, expands each pair via inner `rep` helper,
+  and concatenates with `@`. Companion to run_length encoding from
+  iteration 130. `[(1,3);(2,2);(3,4);(1,2)]` expands to
+  [1;1;1;2;2;3;3;3;3;1;1] (sum = 21). 81 baseline programs total.
 - 2026-05-09 Phase 5.1 — peano.ml baseline (Peano arithmetic, 5*6 =
   30). Defines `type peano = Zero | Succ of peano` and four
   recursive functions: to_int, from_int, plus, mul. Multiplication