Function decode_single_packet_passband 

pub fn decode_single_packet_passband(
    pkt_audio: &[f32],
    cfg: &OfdmConfig,
) -> Option<Vec<u8>>

Decodes one passband packet waveform into payload bytes.

Parameters:

• pkt_audio: passband packet waveform (wake + guard + OFDM body).
• cfg: modem configuration. Returns:
• Option<Vec<u8>>: decoded payload bytes, or None on failure.