A screen showing only a spinner has nothing focusable, so a slow or hung
extraction stranded the tvOS remote. Add TVStalledLoadCancelButton, a
Cancel that appears under a loading spinner after three seconds so
normal loads never flash it, and use it for external video extraction.
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.
