Add ERC-777 reentrancy check template

[MyTH-zyxeon]

Summary

• add ERC777ReentrancyCheck for tokensReceived callback reentrancy during token withdrawals
• add mock ERC-777 token, safe vault, vulnerable vault, and focused Foundry coverage
• document the new check in the README checks table and architecture list

Closes #19

Validation

• forge fmt --check src/checks/ERC777ReentrancyCheck.sol test/ERC777Reentrancy.t.sol
• forge test --match-path test/ERC777Reentrancy.t.sol -vvv (4 passed)
• git diff --check

Note: full forge test still has pre-existing failures in test/Example.t.sol, test/GovernanceExample.t.sol, and test/ERC4626AdvancedCheck.t.sol; the new ERC-777 suite passes independently.

[abhicris]

🤖 Audit verdict: safe

Legitimate security audit template for ERC-777 reentrancy detection with correct vulnerable/safe test examples and no malicious payloads, credential leakage, or supply-chain risks.

Audited by the kcolbchain PR pipeline. See pipeline docs.