Function pr_refresh_due 

pub fn pr_refresh_due(diff: &GitDiffSnapshot, now: SystemTime) -> bool

Whether a scan’s PR block is past its refetch window, so a scan that may reach the network would look it up again.

The seat’s watch is the scan that refreshes it: the read path serves what the store holds, and this is the leg that makes the watch’s next pass scan even when the tree itself is still - otherwise the correction would wait for the tree’s own staleness window (10 s) rather than its own (~1 s).