Function read_rows_with 

fn read_rows_with(
    config_dir: &Path,
    session_id: &str,
    directory: Option<&str>,
    below: Option<&TranscriptAnchor>,
    rows_wanted: usize,
    step: u64,
    cap: u64,
) -> Option<TranscriptSpan>

read_rows with the page’s own budget in the caller’s hands, so the growth, the cap and the search bound can be driven by a test without a transcript of the size the production budget is written for.