Royalty Enforcement Checker

Reads royaltyInfo() for a given token and sale price, and checks whether the contract properly declares EIP-2981 through supportsInterface — a collection can answer one and not the other, and marketplaces that check the interface first will pay nothing. Flags zero recipients and nil rates. Says plainly that EIP-2981 is a hint rather than enforcement: it tells a marketplace what to pay, and nothing in it compels payment. This reads what a collection declares, which is an upper bound on what is actually paid. Marketplaces decide independently whether to honour EIP-2981, and most now treat it as optional. A collection can answer royaltyInfo without declaring the interface, in which case many marketplaces will pay nothing.