ocaml: phase 5.1 max_run.ml baseline (longest consecutive run, 4+1+0 = 5)

Walks list keeping a previous-value reference; increments cur on
match, resets to 1 otherwise. Uses 'Some y when y = x' guard pattern
in match for the prev-value comparison:

  let max_run xs =
    let max_so_far = ref 0 in
    let cur = ref 0 in
    let last = ref None in
    List.iter (fun x ->
      (match !last with
       | Some y when y = x -> cur := !cur + 1
       | _ -> cur := 1);
      last := Some x;
      if !cur > !max_so_far then max_so_far := !cur
    ) xs;
    !max_so_far

Three test cases:
  [1;1;2;2;2;2;3;3;1;1;1]   max run = 4 (the 2's)
  [1;2;3;4;5]                max run = 1
  []                          max run = 0
  sum = 5

Tests 'when' guard pattern in match arm + Option ref + ref-mutation
sequence inside List.iter closure body.

94 baseline programs total.

plans/ocaml-on-sx.md

@@ -407,6 +407,16 @@ _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 — max_run.ml baseline (longest consecutive
+  run, sum of three test cases = 4+1+0 = 5). Walks list with
+  `Some y when y = x` guard pattern in match for the prev-value
+  comparison; runs of equal elements increment cur, resets to 1
+  otherwise. Tests three inputs:
+    [1;1;2;2;2;2;3;3;1;1;1]   max run = 4 (the 2's)
+    [1;2;3;4;5]               max run = 1
+    []                         max run = 0
+  Sum = 5. Tests `when` guard in match arm + Option ref. 94 baseline
+  programs total.
 - 2026-05-09 Phase 5.1 — subseq_check.ml baseline (string is
   subsequence?, 3/5 yes). Two-pointer walk: advance `i` only on
   match, always advance `j`. Match if `i` reaches `n` (consumed