ocaml: phase 5.1 bracket_match.ml baseline (5/9 balanced strings)

Stack-based multi-bracket parenthesis matching for ( [ { ) ] }.
Non-bracket chars are skipped (treated as content).

Tests:
  ()           yes
  [{()}]       yes
  ({[}])       no  (mismatched closer)
  ""           yes
  ((           no  (unclosed)
  ()[](){}     yes
  (a(b)c)      yes  (a/b/c skipped)
  (()          no
  ])           no
                 5 balanced

Body uses begin/end-wrapped match inside while:

  else if c = ')' || c = ']' || c = '}' then begin
    match !stack with
    | [] -> ok := false
    | top :: rest ->
      let pair =
        (c = ')' && top = '(') ||
        (c = ']' && top = '[') ||
        (c = '}' && top = '{')
      in
      if pair then stack := rest else ok := false
  end

Tests side-effecting match arms inside while body, ref-of-list as
stack, multi-char pairing dispatch.

174 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.
+- 2026-05-11 Phase 5.1 — bracket_match.ml baseline (multi-bracket
+  parenthesis matching over 9 test strings, 5 balanced). Stack-
+  based scan: push openers `( [ {`, pop and pair-check closers.
+  Sequence: `() [{()}] ({[}]) "" (( ()[](){} (a(b)c) (() ])` →
+  yes/yes/no/yes/no/yes/yes/no/no = 5.
+  Tests `begin … match !stack with [] -> … | top :: rest -> … end`
+  with side-effecting match arms inside a `while` body, ref-of-list
+  used as a stack, three-way char dispatch via short-circuited
+  comparisons. 174 baseline programs total.
 - 2026-05-11 Phase 5.1 — wildcard_match.ml baseline (recursive `*`
   /`?` wildcard matcher; over 3×6 = 18 (pattern, text) combos,
   6 match). Cases: