Function opens_for_debt 

pub fn opens_for_debt(scan: &mut TurnScan, message: &Message) -> bool

Whether a frame opens a turn for a reader that attributes counters to the newest turn: the scan’s own openings plus the two shapes the fold opens at and the window cut deliberately cannot name - a queued prompt and a delivery’s envelope row.

The cut’s under-approximation is safe; the debt’s is not. A cut the scan cannot start on leaves a turn’s head in place, while a debt attributed to the wrong turn adds another turn’s counters to the figure.