Two near-end checkpoints arrive together routinely (the 10 s tick and the
debounced seeked flush); both read "not watched" before writing and both
scrobbled, bumping Plex's viewCount twice. The transition is now decided by
the write: an insert that did not conflict, or an update conditional on the
old status.
The finish also popped the push-ordering entry that _resume_push_superseded
reads, so an older resume push still in flight could land at Plex after the
scrobble and revive a resume point on a watched item. It now stamps the
entry instead; the existing eviction bounds the map.
