fix: Backtesting #2: Foundry script skeleton + Uniswap V3 shadow pool deployment (#316)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

onchain/script/backtesting/MockToken.sol

@@ -0,0 +1,24 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+pragma solidity ^0.8.19;
+
+import { ERC20 } from "@openzeppelin/token/ERC20/ERC20.sol";
+
+/**
+ * @title MockToken
+ * @notice Minimal ERC20 with open mint for backtesting only. No access control.
+ */
+contract MockToken is ERC20 {
+    uint8 private _dec;
+
+    constructor(string memory name, string memory symbol, uint8 decimals_) ERC20(name, symbol) {
+        _dec = decimals_;
+    }
+
+    function decimals() public view override returns (uint8) {
+        return _dec;
+    }
+
+    function mint(address to, uint256 amount) external {
+        _mint(to, amount);
+    }
+}