Delete declarations that Periphery reports unused on every platform that
compiles them (iOS, macOS, tvOS), including nine views used only by their
own previews. Platform-gated code still used on another platform, device-
only MPV render paths, migration models, CloudKit record names, App
Intents metadata and unused parameters are kept.
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.
