Custom Error Selector Generator

Matches a selector from a failed call against candidate error signatures, and generates selectors from a list of declarations. Signatures are canonicalised first — uint written out as uint256, parameter names removed — because hashing a declaration exactly as it appears in Solidity produces a different selector, which is the usual reason a lookup fails against a perfectly correct ABI. The two compiler-emitted errors are recognised directly, including the full panic code table, since those never appear in any ABI and people spend a long time looking for them there. Errors in your own list that collide on four bytes are flagged, because a caller cannot tell those apart.