ocaml: phase 5.1 balance.ml baseline (paren/bracket/brace balance via Stack)

is_balanced walks a string; on each char:
  '(', '[', '{'  -> Stack.push c
  ')', ']', '}'  -> require stack non-empty AND top = expected opener,
                     else mark ok = false
  others         -> skip

At end: !ok && Stack.is_empty stack.

Five test cases:
  '({[abc]d}e)'  -> true
  '(a]'           -> false  (no matching opener)
  '{[}]'          -> false  (mismatched closer)
  '(())'          -> true
  ''              -> true

Sum of (if balanced then 1 else 0) -> 3.

Exercises:
  Stack.create / push / pop / is_empty
  s.[!i] string indexing
  while loop + bool ref short-circuit
  multi-arm if/else if/else if dispatch

31 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 — balance.ml baseline (paren/bracket/brace
+  balance using Stack). is_balanced walks a string; on opener push,
+  on closer check stack non-empty + top matches expected opener (else
+  fail). Returns ok && is_empty stack at end. 5 test cases:
+  "({[abc]d}e)" ✓, "(a]" ✗, "{[}]" ✗ (mismatched closers), "(())" ✓,
+  "" ✓ → 3 balanced. Exercises Stack.create / push / pop / is_empty /
+  s.[!i] / while + bool ref short-circuit. 31 baseline programs total.
 - 2026-05-09 Phase 5.1 — safe_div.ml baseline + Result.equal /
   compare / iter_error (+3 tests, 592 total). safe_div divides only
   if divisor non-zero, returns `Error "..."` otherwise. sum_safe folds