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.

lib/ocaml/baseline/expected.json

@@ -1,6 +1,7 @@
 {
   "anagrams.ml": 3,
   "bag.ml": 3,
+  "balance.ml": 3,
   "bfs.ml": 6,
   "btree.ml": 39,
   "brainfuck.ml": 75,