Function write_tail_payload 

pub fn write_tail_payload(
    data: &mut [u8],
    body_end: usize,
    payload: &[u8],
) -> Result<usize, ProgramError>

Write an already-encoded dynamic-tail payload.

This is used by generated bare-final-tail accounts: bounded fields are encoded first, then TailStr / TailBytes contributes the remaining raw payload without its own field-level length prefix.