feat: add V1 custody accounting

src/BankV1.sol

@@ -2,6 +2,7 @@
 pragma solidity 0.8.35;
 
 import {IERC20} from "@openzeppelin/contracts/token/ERC20/IERC20.sol";
+import {SafeERC20} from "@openzeppelin/contracts/token/ERC20/utils/SafeERC20.sol";
 import {Initializable} from "@openzeppelin/contracts/proxy/utils/Initializable.sol";
 import {UUPSUpgradeable} from "@openzeppelin/contracts/proxy/utils/UUPSUpgradeable.sol";
 import {ReentrancyGuardTransient} from "@openzeppelin/contracts/utils/ReentrancyGuardTransient.sol";
@@ -9,7 +10,15 @@ import {OwnableUpgradeable} from "@openzeppelin/contracts-upgradeable/access/Own
 import {PausableUpgradeable} from "@openzeppelin/contracts-upgradeable/utils/PausableUpgradeable.sol";
 
 contract BankV1 is Initializable, UUPSUpgradeable, OwnableUpgradeable, PausableUpgradeable, ReentrancyGuardTransient {
+    using SafeERC20 for IERC20;
+
     error InvalidAsset(address asset);
+    error ZeroAmount();
+    error InsufficientBalance(address account, uint256 available, uint256 requested);
+    error UnexpectedAssetDelta(uint256 expected, uint256 actual);
+
+    event Deposited(address indexed account, uint256 amount);
+    event Withdrawn(address indexed account, uint256 amount);
 
     IERC20 internal _asset;
     mapping(address account => uint256 balance) internal _balances;
@@ -40,6 +49,31 @@ contract BankV1 is Initializable, UUPSUpgradeable, OwnableUpgradeable, PausableU
         _unpause();
     }
 
+    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);
+    }
+
+    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);
+    }
+
     function asset() external view returns (IERC20) {
         return _asset;
     }