data: URI scheme for DASH manifests is unreliable in ExoPlayer.
Write MPD XML to FileSystem.cacheDirectory/bili_dash.mpd and pass
file:// URI to react-native-video instead. URL resolution is now
async via useEffect+state in NativeVideoPlayer.
- getPlayUrl uses fnval=16 (DASH) on Android, keeping fnval=0/html5 for iOS/web
- New utils/dash.ts builds a valid DASH MPD from Bilibili's segmentBase ranges
and returns it as a data: URI for ExoPlayer consumption
- NativeVideoPlayer selects DASH path (type='mpd') or durl fallback automatically
- Extend PlayUrlResponse types with DashVideoItem/DashAudioItem/DashSegmentBase
