VideoAds while the current one plays, prefetch its ads, and destroy the current one when the viewer leaves. Any number can be alive at once; the SDK keeps at most one ad on screen in the whole app.
track(_:) when the episode actually starts in the player, not at creation: before that the player’s time belongs to the previous episode.
For a post-roll, show this episode’s linear ad when it finishes, and move on only from its didEnd: destroy() ends the ad on screen too, a linear ad included.