Function pin_wall 

pub fn pin_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.

Only a capture takes this: it draws every frame of the process from the thread that pins it here, so it needs the whole process, not only its own thread. A test holds its own thread with [testing::with_wall], or, when the time it pins must reach its own helper threads too, takes the process with tilo_test_env::in_own_process and [testing::hold_wall_everywhere].