Three MediaBrowser pagination loops re-implemented drainPages; it gains an
onPage hook (fires after intermediate pages only) so the two
progressive-render loops fit as well as the episode queue. Five
handwritten FIFO future tails (companion lifecycle, Seerr persistence,
tracker write lock, companion host-auth commit, Watch Together message
routing) become SerialFutureQueue, and the companion peer service's
disconnect/dispose in-flight joins become FutureCoalescer.
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