When a series episode reaches 30s before its end, the player enters
post-roll and pauses the underlying video to stop HLS from buffering the
tail segment. A paused video never fires the `ended` event, but the
PlayingNextScreen countdown was gated on `postRollVideoEnded`, which only
became true on `ended`. The result: the "Up Next" overlay appears, the
video is paused, and the countdown never starts — it only resumes if the
user manually plays the video (letting it reach the end and fire `ended`).
Arm the countdown gate at the moment we enter post-roll early, since that
is the point at which we've decided playback is over, rather than waiting
for an `ended` event that an intentionally-paused video will never emit.
