v0.5 used `sleep 5` after a static "System will reboot in 5 seconds."
box, which left the operator guessing how much time was left to grab
the USB stick. The new loop runs 10 → 1, clearing + redrawing the
gum-style success box each tick with the remaining-seconds figure,
giving the operator a visible window to act.
10 seconds (vs 5) because real hardware operators were missing the
window — laptops with the USB on the far side of the dock take
4-5 seconds to physically reach. 10 is comfortable, not annoying.
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