Function prefill_at 

pub fn prefill_at(
    spec: &ModelSpec,
    dir: &str,
    ctx: usize,
    toks: &[usize],
    pos0: usize,
) -> Authored

Cut::Prefill at a run’s own inputs rather than its placeholders: the chunk of toks (token ids from 1) at positions from pos0, its manifest under dir — the text a run executes. Its identity is the cut’s (written), whose inputs sit at their placeholders.