totalFees overflows and truncates due to uint64 math under Solidity ^0.7.6

Description

totalFees is declared uint64 (src/PuppyRaffle.sol:30) and selectWinner accumulates fees with totalFees = totalFees + uint64(fee) (line 134). Under pragma solidity ^0.7.6 (line 2) arithmetic does NOT revert on overflow, and the uint64(fee) cast silently truncates any fee above 2^64 - 1 wei (~18.4 ETH).

Risk

Likelihood:

Medium. A single large raffle (fee > ~18.4 ETH) truncates immediately; otherwise fees accumulate across rounds until the uint64 sum wraps. Both are reachable through normal protocol operation given enough volume, with no attacker interaction required.

Impact:

Medium. totalFees no longer reflects the ETH actually held, corrupting fee accounting. The under-counted value breaks withdrawFees (line 158), which requires address(this).balance == uint256(totalFees); that equality can no longer hold, permanently locking fees in the contract.

Proof of Concept

Accumulating fees past the uint64 ceiling makes totalFees wrap below the real balance.

Recommended Mitigation

Use uint256 for fees (Solidity 0.8+ adds built-in overflow checks; on 0.7.x use SafeMath).