Function is_checkpoint_chunk_end 

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

2026-09-27: Whether a non-last chunk ending at end_token saves an SSM snapshot: a prompt-tail end (the last block boundary under total, or one block below it), or a block boundary whose block index is a multiple of interval (0 turns interval snapshots off). Block 0 never does.