Pings now run every 3 seconds, so a working idle link always delivers a
frame inside the 6 second quick probe window. A probe that a retry
shortened now extends by the retry's window, not the original one. A
mobile refocus without a background visit sends application-refocused,
which restarts the retry ladder and checks for a better route but does
not probe the live session. Adds a test of the read counter against
real pongs.