Document music room design

[han4wluc]

Summary

• define the singleton Music Room catalog and computed projection
• specify locks, pagination, play, pause, stop, seek, and navigation actions
• document renderer events, stale-event guards, audio focus, and lifecycle behavior
• keep the proposal concise and aligned with the Image Gallery architecture

Validation

• bun run prettier docs/MusicBox.md -c
• git diff --check