Function resolve_anchor 

fn resolve_anchor(
    file: &mut File,
    session_id: &str,
    len: u64,
    anchor: &TranscriptAnchor,
    step: u64,
    cap: u64,
) -> Option<u64>

Where a read’s window ends for a named anchor: the byte its row starts at, seeking when the cursor carries a byte and searching from the file’s end when it does not.

None when the anchor cannot be placed - a row the file does not carry, or one further from the end than SPAN_CAP_BYTES reaches - which the caller answers as the end of the history.