Function freeze_wall 

pub fn freeze_wall(at: SystemTime)

Pin the wall clock to at for the rest of the process, so a screenshot capture shows the same date and time on every run. The monotonic clock (now) keeps running: animations and timeouts still advance. Never called in a live session.

The calling thread reads at from then on even when another thread pins another time later, so tests sharing one process each keep the time they chose; other threads read the last time pinned anywhere.