When _process_queue found another worker already processing, it called
_schedule_timer(0), spawning a fresh Timer thread immediately and looping
tightly (spawn -> busy -> reschedule -> spawn) until the active worker
finished. Replace this with a _reprocess_pending flag: a concurrent caller
sets the flag and returns, and the active worker reschedules exactly once
in its finally block when work remains. Reset the flag in clear().
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