Function encode_packet_data 

pub fn encode_packet_data(
    buf: &mut Vec<u8>,
    packet_id: i32,
    encode_fn: impl FnOnce(&mut Vec<u8>),
)

Encode a typed packet into raw bytes (packet ID + fields), without framing.

Clears buf and writes the packet ID followed by the encoded fields into it. The caller should supply a reusable buffer so that its capacity is retained across calls, avoiding per-packet allocations.