Event Topic Builder

Turns an event signature and its indexed positions into a working log filter, with the exact JSON and the matching ethers call. The mistake it exists to prevent is positional: an indexed parameter you are not filtering on must be null rather than left out, because omitting it shifts every later topic up a slot and the filter quietly matches something else entirely. It also refuses more than three indexed parameters, since a log carries four topics and the first is the signature. Dynamic indexed types are handled properly — a string or bytes parameter is indexed as the keccak hash of its value, so filtering on the plain text matches nothing and the original cannot be recovered from the log at all.