Interface BeastFrame

One deframed Beast message. decoded is populated when the raw message decoded successfully - a ModeAcReply for modeAC frames, or a DecodedModeSMessage (from @squawk/mode-s) for shortModeS/longModeS frames whose downlink format and CRC this package's decoder supports. It is undefined for a Mode-S frame that failed its CRC check or whose downlink format isn't decoded - rawMessage is always populated regardless, so a consumer that wants the bytes anyway (for logging, or its own decode attempt) still has them.