Changes restart the stream — the receiver resets automatically (new
session id in the frame headers; that's the fountain protocol's gift).
Max screen brightness helps. The stream loops forever — stop when the
receiver says done.
point the camera at the sender's code
Settings
Set before starting the camera. 1280-wide is the widest mode iOS runs
at a true 60 fps; the fps request is demanded with `exact` first
because `ideal: 60` silently delivers 30.