The S4 Play-all wrapper had no test — a regression inverting the empty-queue
guard or dropping the notice would re-introduce the silent no-op it was added
to fix, and stay green. Adds a PlexCollectionView DOM test (spied api +
notify) asserting: a leaf queues as itself, a show expands to its episodes in
season->episode order, and an empty expansion surfaces plex.collection.playEmpty
instead of no-op'ing.