Function build_packet_bytes 

pub fn build_packet_bytes(
    payload: &[u8],
    frag_index: u8,
    frag_count: u8,
    cfg: &OfdmConfig,
) -> Vec<u8> 

Builds one packet (header + payload + CRC).

Parameters:

• payload: fragment payload bytes.
• frag_index: zero-based fragment index.
• frag_count: total fragment count.
• cfg: modem configuration (session/modulation fields used). Returns:
• Vec<u8>: serialized packet bytes.