Function rows_below_floor 

async fn rows_below_floor(
    state: &TransportState,
    seat: &SessionSlot,
    cwd: &Path,
    below: Option<TranscriptAnchor>,
    turns: u32,
) -> BelowFloor

Answer a page below the held window’s floor from the session’s transcript.

Rows, by their own bytes. The read is handed the row the page above stopped at and answers the rows strictly older than it, so the walk crosses a window whose front is a cut inside a running turn - where the frame numbering is unusable - exactly as it crosses one that ends on a turn. The page is cut, rendered and cursor-written by the same page_within the held window’s pages are, and the cursor it hands back names its own oldest row, so the page below it seeks.

Every case the transcript cannot answer - no file, an anchor the file does not carry, an anchor further from the end than the cap reaches, a read that failed - is BelowFloor::Empty, the page the client already draws as the end of the history rather than an error it cannot draw at all.