### Task 4: Implement V1 custody accounting and adversarial unit tests

**Files:**
- Modify: `src/BankV1.sol`
- Create: `test/BankV1.t.sol`
- Create: `test/mocks/FeeOnTransferToken.sol`
- Create: `test/mocks/ReentrantToken.sol`

**Interfaces:**
- Produces `deposit(uint256)` and `withdraw(uint256)` plus `Deposited`, `Withdrawn`, `ZeroAmount`, `InsufficientBalance`, and `UnexpectedAssetDelta`.
- Consumes only a standard non-rebasing ERC-20 through `IERC20`/`SafeERC20`.
- Preserves `reserves >= totalLiabilities`; direct token transfers may create surplus.

- [ ] Write event and error declarations into the test expectations before production code:

```solidity
event Deposited(address indexed account, uint256 amount);
event Withdrawn(address indexed account, uint256 amount);

error ZeroAmount();
error InsufficientBalance(address account, uint256 available, uint256 requested);
error UnexpectedAssetDelta(uint256 expected, uint256 actual);
```

- [ ] Write failing deposit tests for exact balance/liability/reserve deltas, event emission, zero amount, paused state, inadequate allowance, inadequate wallet balance, and two independent customers. Use six-decimal constants (`1_000e6`) so assertions read like the demo.

- [ ] Add `FeeOnTransferToken` whose `transferFrom` delivers `99%` of the requested amount. Assert a deposit reverts with `UnexpectedAssetDelta(requested, received)` and that the ERC-20 transfer, bank balance, and liabilities are all rolled back.

- [ ] Add `ReentrantToken` that attempts a nested `bank.deposit` during `transferFrom`. Assert the nested call receives `ReentrancyGuardReentrantCall` and the outer call either completes once or reverts atomically according to the mock’s configured propagation mode; no double credit is permitted.

- [ ] Write failing withdrawal tests for exact deltas, event emission, zero amount, paused state, insufficient internal balance (including available/requested values), checks-effects-interactions under a transfer callback, and one account’s withdrawal leaving another account unchanged.

- [ ] Write the surplus test: deposit 100 mUSDC, transfer 25 mUSDC directly to the proxy, assert reserves `125e6`, liabilities `100e6`, and a normal 100 mUSDC withdrawal succeeds while the 25 mUSDC surplus remains. Assert there is no owner sweep/rescue behavior by keeping such a function out of the interface.

- [ ] Run red tests:

```bash
npm_config_offline=true forge test --match-path test/BankV1.t.sol -vvv --force
```

Expected red: `deposit`/`withdraw` and their errors/events are absent.

- [ ] Implement `deposit` with exact received-amount validation and state credit only after a successful transfer:

```solidity
function deposit(uint256 amount) external whenNotPaused nonReentrant {
    if (amount == 0) revert ZeroAmount();

    uint256 reservesBefore = _asset.balanceOf(address(this));
    _asset.safeTransferFrom(msg.sender, address(this), amount);
    uint256 reservesAfter = _asset.balanceOf(address(this));
    uint256 received = reservesAfter >= reservesBefore ? reservesAfter - reservesBefore : 0;
    if (received != amount) revert UnexpectedAssetDelta(amount, received);

    _balances[msg.sender] += amount;
    _totalLiabilities += amount;
    emit Deposited(msg.sender, amount);
}
```

- [ ] Implement `withdraw` with checks-effects-interactions:

```solidity
function withdraw(uint256 amount) external whenNotPaused nonReentrant {
    if (amount == 0) revert ZeroAmount();
    uint256 available = _balances[msg.sender];
    if (amount > available) revert InsufficientBalance(msg.sender, available, amount);

    _balances[msg.sender] = available - amount;
    _totalLiabilities -= amount;
    _asset.safeTransfer(msg.sender, amount);
    emit Withdrawn(msg.sender, amount);
}
```

- [ ] Add fuzz tests in `BankV1.t.sol`: bound deposit to `[1, 1_000_000e6]`; bound withdrawal to `[1, deposited]`; prove exact reserve/liability/customer deltas and that over-withdraw always reverts. Keep a fixed seed in `foundry.toml` for presentation reproducibility while printing Foundry’s replay seed on failure.

- [ ] Run focused and aggregate green tests:

```bash
forge fmt
npm_config_offline=true forge test --match-path test/BankV1.t.sol -vvv --force
npm_config_offline=true forge test --match-path 'test/BankV1*.t.sol' --force
```

Expected: all V1 tests pass with validated proxy deployment enabled.

- [ ] Commit:

```bash
git add src/BankV1.sol test/BankV1.t.sol test/mocks/FeeOnTransferToken.sol test/mocks/ReentrantToken.sol
git commit -m "feat: add V1 custody accounting"
```

---