PacketQueue::countBefore() and PacketQueue::get() use unsigned
comparison (_schedule_table[j] > now) to check if a packet is
scheduled for the future. This breaks when millis() wraps around
after ~49.7 days: packets scheduled just before the wrap appear
to be in the far future and get stuck in the queue.
Use signed subtraction instead, matching the approach already used
by Dispatcher::millisHasNowPassed(). This correctly handles the
wraparound for time differences up to ~24.8 days in either
direction, well beyond the maximum queue delay of 32 seconds.
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