feat: demonstrate state-preserving V2 upgrade

test/helpers/BankV2Handler.sol

@@ -0,0 +1,32 @@
+// SPDX-License-Identifier: MIT
+pragma solidity 0.8.35;
+
+import {BankV2} from "../../src/BankV2.sol";
+import {MockUSDC} from "../../src/MockUSDC.sol";
+import {BankHandler} from "./BankHandler.sol";
+
+contract BankV2Handler is BankHandler {
+    BankV2 internal immutable bankV2;
+
+    uint256 public ghostTransferred;
+
+    constructor(MockUSDC token_, BankV2 bank_) BankHandler(token_, bank_) {
+        bankV2 = bank_;
+    }
+
+    function transfer(uint256 fromSeed, uint256 toSeed, uint256 amount) external {
+        uint256 count = actorCount();
+        uint256 fromIndex = fromSeed % count;
+        address from = actorAt(fromIndex);
+        uint256 balance = bankV2.balanceOf(from);
+        if (balance == 0) return;
+
+        uint256 toIndex = (fromIndex + 1 + (toSeed % (count - 1))) % count;
+        address to = actorAt(toIndex);
+        amount = bound(amount, 1, balance);
+
+        vm.prank(from);
+        bankV2.transferBalance(to, amount);
+        ghostTransferred += amount;
+    }
+}