feat: add UpgradeToV2 deployment script

Forge script for upgrading existing SplitBaseV1 proxies to V2:
- Deploys new SplitBaseV2 implementation
- Executes upgradeToAndCall with initializeV2
- Outputs verification command for Basescan
- Maintains storage compatibility with V1

Usage:
PROXY_ADDRESS=0x... forge script script/UpgradeToV2.s.sol \
  --rpc-url base_sepolia --broadcast --verify

Author: katyailil

script/UpgradeToV2.s.sol

@@ -0,0 +1,37 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.28;
+
+import {Script, console} from "forge-std/Script.sol";
+import {SplitBaseV2} from "../src/SplitBaseV2.sol";
+
+contract UpgradeToV2Script is Script {
+    function run() external {
+        uint256 deployerPrivateKey = vm.envUint("PRIVATE_KEY");
+        address proxyAddress = vm.envAddress("PROXY_ADDRESS");
+
+        console.log("Upgrading to SplitBaseV2...");
+        console.log("Proxy address:", proxyAddress);
+        console.log("Deployer:", vm.addr(deployerPrivateKey));
+
+        vm.startBroadcast(deployerPrivateKey);
+
+        SplitBaseV2 newImplementation = new SplitBaseV2();
+        console.log("SplitBaseV2 implementation deployed at:", address(newImplementation));
+
+        SplitBaseV2 proxy = SplitBaseV2(proxyAddress);
+        proxy.upgradeToAndCall(address(newImplementation), abi.encodeCall(proxy.initializeV2, ()));
+        console.log("Upgrade to V2 complete");
+
+        vm.stopBroadcast();
+
+        console.log("\nVerification command:");
+        console.log(
+            "forge verify-contract",
+            address(newImplementation),
+            "src/SplitBaseV2.sol:SplitBaseV2",
+            "--chain-id",
+            block.chainid,
+            "--watch"
+        );
+    }
+}