Function write_group 

fn write_group<B: ByteBuffer>(
    out: &mut PacketWriter<B>,
    max_packet_size: Option<u32>,
    packet_type: PacketType,
    packet_id: PacketId,
    props_content: usize,
    payload: usize,
    encode_body: impl FnOnce(&mut Writer<'_>) -> Result<(), EncodeError>,
) -> Encoded

Frames a SUBSCRIBE/UNSUBSCRIBE group and writes it in one piece: fixed header (§3.8.1/§3.10.1 mandate flags 0b0010), packet id, properties section, then the caller’s payload.