ocaml: phase 5.1 atm.ml baseline (mutable record + exception + try/with, balance = 120)

Models a bank account using a mutable record + a user exception:

  type account = { mutable balance : int }
  exception Insufficient

  let withdraw acct amt =
    if amt > acct.balance then raise Insufficient
    else acct.balance <- acct.balance - amt

Sequence:
  start             100
  deposit 50        150
  withdraw 30       120
  withdraw 200      raises Insufficient
  handler returns   acct.balance        (= 120, transaction rolled back)

Combines mutable record fields, user exception declaration,
try-with-bare-pattern, and verifies that a raise in the middle of a
sequence doesn't leave a partial mutation.

59 baseline programs total.

lib/ocaml/baseline/atm.ml

@@ -0,0 +1,17 @@
+type account = { mutable balance : int }
+
+exception Insufficient
+
+let withdraw acct amt =
+  if amt > acct.balance then raise Insufficient
+  else acct.balance <- acct.balance - amt
+
+let deposit acct amt = acct.balance <- acct.balance + amt
+
+;;
+
+let a = { balance = 100 } in
+deposit a 50;
+withdraw a 30;
+try (withdraw a 200; -1)
+with Insufficient -> a.balance

lib/ocaml/baseline/expected.json

@@ -2,6 +2,7 @@
   "ackermann.ml": 125,
   "anagram_check.ml": 2,
   "anagrams.ml": 3,
+  "atm.ml": 120,
   "bag.ml": 3,
   "bf_full.ml": 6,
   "bigint_add.ml": 28,

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.
+- 2026-05-09 Phase 5.1 — atm.ml baseline (mutable record + custom
+  exception + try/with, balance 120 after rollback). Models a bank
+  account: deposit/withdraw mutate the balance field; an over-draw
+  raises `Insufficient` which the caller catches and falls back to
+  the unchanged balance. Starting at 100, +50 = 150, -30 = 120,
+  attempted -200 throws and the handler returns 120. Combines
+  mutable record fields, user exception declaration, and
+  try-with-bare-pattern in a realistic micro-pattern. 59 baseline
+  programs total.
 - 2026-05-09 Phase 5.1 — bf_full.ml baseline (Brainfuck interpreter
   with `[`/`]` loops, `+++[.-]` → 3+2+1 = 6). Extends the iter-92
   brainfuck.ml subset with bracket matching: `[` jumps past matching