Two session requests for one item in flight at once (a quick double seek; the
StrictMode replay of the start effect) let the server's last spawn be the older
request's. The newer request's answer then named a session already gone; its
playlist 404'd before a manifest parsed, and the player stopped with "Playback
couldn't start". The 404 reclaim now covers a session that never played too,
bounded by the same budget, and keeps the start's own autoplay.