feat: add valueless mock USDC

src/MockUSDC.sol

@@ -0,0 +1,18 @@
+// SPDX-License-Identifier: MIT
+pragma solidity 0.8.35;
+
+import {ERC20} from "@openzeppelin/contracts/token/ERC20/ERC20.sol";
+import {Ownable} from "@openzeppelin/contracts/access/Ownable.sol";
+
+/// @notice Educational mock token with no monetary value. Never use as real USDC.
+contract MockUSDC is ERC20, Ownable {
+    constructor(address initialOwner) ERC20("Mock USD Coin", "mUSDC") Ownable(initialOwner) {}
+
+    function decimals() public pure override returns (uint8) {
+        return 6;
+    }
+
+    function mint(address to, uint256 amount) external onlyOwner {
+        _mint(to, amount);
+    }
+}

test/MockUSDC.t.sol

@@ -0,0 +1,60 @@
+// SPDX-License-Identifier: MIT
+pragma solidity 0.8.35;
+
+import {Test} from "forge-std/Test.sol";
+import {Ownable} from "@openzeppelin/contracts/access/Ownable.sol";
+import {MockUSDC} from "../src/MockUSDC.sol";
+
+contract MockUSDCTest is Test {
+    MockUSDC internal token;
+    address internal constant STRANGER = address(0xBEEF);
+    address internal constant RECIPIENT = address(0xCAFE);
+    address internal constant SPENDER = address(0xD00D);
+
+    function setUp() public {
+        token = new MockUSDC(address(this));
+    }
+
+    function testMetadataUsesSixDecimalMockUSDC() public view {
+        assertEq(token.name(), "Mock USD Coin");
+        assertEq(token.symbol(), "mUSDC");
+        assertEq(token.decimals(), 6);
+    }
+
+    function testInitialSupplyIsZero() public view {
+        assertEq(token.totalSupply(), 0);
+    }
+
+    function testMintMintsToRecipientWhenCalledByOwner() public {
+        token.mint(RECIPIENT, 1_250_000);
+
+        assertEq(token.totalSupply(), 1_250_000);
+        assertEq(token.balanceOf(RECIPIENT), 1_250_000);
+    }
+
+    function testMintRevertsWhenCalledByNonOwner() public {
+        vm.prank(STRANGER);
+        vm.expectRevert(abi.encodeWithSelector(Ownable.OwnableUnauthorizedAccount.selector, STRANGER));
+        token.mint(RECIPIENT, 1);
+    }
+
+    function testTransferMovesMintedBalance() public {
+        token.mint(address(this), 1_250_000);
+
+        token.transfer(RECIPIENT, 250_000);
+
+        assertEq(token.balanceOf(address(this)), 1_000_000);
+        assertEq(token.balanceOf(RECIPIENT), 250_000);
+    }
+
+    function testApproveSetsAllowanceForSpender() public {
+        token.approve(SPENDER, 750_000);
+
+        assertEq(token.allowance(address(this), SPENDER), 750_000);
+    }
+
+    function testConstructorRevertsForZeroInitialOwner() public {
+        vm.expectRevert(abi.encodeWithSelector(Ownable.OwnableInvalidOwner.selector, address(0)));
+        new MockUSDC(address(0));
+    }
+}