Function encode_single_packet_passband 

pub fn encode_single_packet_passband(
    payload: &[u8],
    cfg: &OfdmConfig,
) -> Vec<f32>

Encodes one payload fragment (single packet) into passband samples.

Parameters:

• payload: single-packet payload bytes (must fit packet payload budget).
• cfg: modem configuration. Returns:
• Vec<f32>: passband waveform for one packet.