Function is_prompt_tail_end 

pub fn is_prompt_tail_end(
    end_token: usize,
    total: usize,
    block_size: usize,
) -> bool

2026-09-27: Whether a chunk end is a prompt-tail checkpoint: the last block boundary under total, or one block below it (tail_split_point). A warm next turn’s block-floored match lands at one of the two.