Use Custom errors instead of string-based require statements

The MatChain Genesis License protocol uses string-based require statements for error handling:

require($._claimQueueIds.contains(index_), "Invalid Claim request");
require(to_ == msg.sender, "Invalid recipient");

This approach is less gas-efficient than custom errors (available since Solidity 0.8.4), provides limited context for debugging, and lacks ABI integration for frontends.

Implement custom errors throughout the protocol:

// In interfaces
error ClaimNotInActiveQueue(uint256 claimId);
error InvalidRecipient(address expected, address actual);

// In implementation
if (!$._claimQueueIds.contains(index_)) {
    revert ClaimNotInActiveQueue(index_);
}

This change would reduce gas costs, improve error clarity with dynamic values, and enhance integration with development tools.