Function ssm_tail_boundary 

pub fn ssm_tail_boundary(
    total_tokens: usize,
    block_size: usize,
) -> Option<usize>

2026-09-25: The last multiple of block_size strictly below total_tokens, or None when block_size is 0 or total_tokens <= block_size.

The SSM tail snapshot is placed here: in-pass by prepare_midchunk_capture (metrale-model-engine), or, with ssm_tail_ckpt_enabled on and ssm_tail_midchunk_enabled off, by ending a prefill chunk here (run_standard and run_batched_mixed in metrale-server).