Function encode_single_packet_passband_body 

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

Encodes one payload fragment into passband OFDM-body samples only.

Parameters:

• payload: single-packet payload bytes (must fit packet payload budget).
• cfg: modem configuration. Returns:
• Vec<f32>: passband waveform for the OFDM body only, without wake/guard.