collect() cached queue emptiness across sweeps, so a worker's last
write could land after the final sweep but before shutdown was
observed, stranding that frame's clusters. Read the stop flags before
sweeping instead: once workers are joined, a sweep that moves nothing
proves the queues are drained.
Found via ClusterFinderCUDA.ipynb's drift plot, which jumped by one
frame's worth of clusters at the very end.
kferjaoui
f4bbbc9c76