Drop the dashEnabled toggle and its plumbing. Stream auto-selection now
keeps DASH as a candidate but ranks it strictly below progressive and
HLS formats, so DASH is only chosen when it is the only format available
(including live-stream fallback after HLS). The manual quality selector
hides DASH entirely on iOS/tvOS/macOS. Reconciles BackendSwitcher, which
previously treated HLS/DASH equally, to use the same DASH-last ranking.
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