Cover all previously-untested session handlers (authenticate,
handleDSGN, handleWIIUSGN, handlePSSGN, handlePSNLink, sendCode)
plus validateLogin ban/bcrypt paths, registerPsnToken, and
sanitizeAddr/startSignCapture. Uses a spyConn implementing
network.Conn to capture plaintext packets for assertion.
Raises signserver coverage from 46% to 63%.