[good first issue] Add ERC-777 reentrancy test template (tokensReceived hook)

[abhicris]

Context

src/checks/ReentrancyCheck.sol + test/Example.t.sol cover classic ERC-20 reentrancy. We do not have a template for ERC-777's tokensReceived reentrancy vector (the same class of bug that drained imBTC/Uniswap V1 in 2020). Auditors importing this checklist currently have to write that template themselves.

Scope

Files to touch:

• Add src/checks/ERC777ReentrancyCheck.sol — parallel to ReentrancyCheck.sol.
• Add test/ERC777Reentrancy.t.sol — includes a deliberately-vulnerable VulnerableERC777Vault.sol fixture and asserts the check flags it.

Acceptance criteria

• Check compiles; test passes when run against the included vulnerable fixture; test fails (= check detects bug) when fixture's nonReentrant guard is removed.
• Documented in README.md under the checks table.

Estimated effort

M — 3-4 hrs. Reference: OZ IERC777Recipient and 2020 Uniswap V1 post-mortem.

— kcolbchain / Abhishek Krishna

[MyTH-zyxeon]

Submitted PR #27 for the ERC-777 reentrancy template: #27

It adds ERC777ReentrancyCheck, a mock ERC-777 token, safe/vulnerable vault fixtures, focused tests, and README coverage. Local validation: forge test --match-path test/ERC777Reentrancy.t.sol -vvv -> 4 passed; fmt and diff checks passed.