Add singleton music room playback

[han4wluc]

Summary

• add the strict singleton resources.musicRoom catalog and computed musicRoom projection
• add paginated track navigation plus play, pause, stop, seek, previous/next, and clear actions
• integrate Route Graphics 1.31.1 command-controlled playback and cursor-preserving looping-BGM audio focus
• route ready, progress, completion, and error events through stale-safe private engine actions
• keep player state transient across saves and rollback while resetting it on load, story reset, and project replacement
• preserve template-looking authored and event-supplied track IDs as literal runtime data

Validation

• bun run test — 1,298 tests passed
• bun run lint
• node esbuild.js
• browser fixture: real canvas Play/Pause/Seek/Resume/Clear dispatch, renderer readiness/progress, stable paused cursor, mixer rerender command retention, and looping-BGM restoration