Split the screen into two registered panel apps that coordinate via a
'music:cwd' panel channel, like /chat:
- music-browser (left): library selector, publishes the path.
- music-detail (right): renders the path — album tracklist, artist discography
sections, or a folder grid — and drives the app-wide player.
MusicScreen is now a WorkspaceView over a horizontal 2-panel layout (persisted as
screens/music), so the panels are resizable. Registered in AppRegistry.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>