A stalled event loop can leave the settle wake-up and a trailing observation
callback due at the same resume. The earlier wake-up runs first, declares
silence, and the capture becomes immutable before the trailing callback
appends its observation.
Drain the loop turn after the quiet sleep so timer and poll callbacks that
were already due run before silence is judged, and cover the stalled-loop
case with a deterministic regression test.
Fixes#1697
Jason Cat
ed084b3dbb