Android Auto: MediaBrowserService shelves for the car

[cubman3134]

#138 parked this explicitly — "Deliberately not in this issue — it needs its own review pass" — while making sure its MediaSession design wouldn't preclude it. Symfonium's well-regarded Android Auto support is the demand evidence, and #138's session work is the prerequisite that makes this mostly presentation.

Proposed behaviour

• A MediaBrowserService exposing a browsable tree Auto renders itself: Continue listening, Audiobooks (Local audiobook library: authors, narrators, series, and multi-file books #139), Artists / Albums / Playlists (Local music library: tag-based album/artist browsing for your own files #74, and the merged view from One music library across every source: merge local, Subsonic, Jellyfin, and server items into unified artists and albums #194 once it exists), Podcasts, and Downloaded — the last one mattering more than the rest, since car connectivity is unreliable and offline content is the reliable answer.
• Depth and size discipline: Auto enforces limits on node counts and nesting, and its review process cares about them. Paginate long lists, cap depth at three levels, and prefer a "Downloaded" or "Recent" shelf over an 8,000-item artist list nobody can safely scroll at speed.
• Voice: support onPlayFromSearch so "play " works — this is the interaction people actually use in a car, and it's a small handler over existing search.
• Playback is Background audio on mobile: keep playing with the screen off, with MediaSession controls #138's, unchanged: the same foreground service, the same MediaSession, the same mpv instance. Auto attaches to what already exists rather than introducing a second playback path.
• Distraction-optimized behaviour: no custom UI (Auto draws it), no dialogs, no error toasts — failures degrade to a spoken/silent skip rather than anything demanding attention. Same reasoning as the lock control in Video player touch gestures: volume, brightness, seek, and skip #162.
• Android Automotive OS (built-in car head units, distinct from phone-projected Auto) is a possible extension since it uses the same media APIs, but has its own distribution and testing story — note it, don't scope it.

Ship criteria: Google's Auto quality checklist passes on a real head unit or the Desktop Head Unit emulator — this is one of the few features where an external review gate exists, so building to it from the start is cheaper than retrofitting.

Not in scope: video (Auto forbids it while driving, correctly), games, and any EverythingBox UI in the car beyond what MediaBrowser provides.