fix(bench/next-beats): remove unused vars flagged by lint
Removes 5 no-unused-vars eslint errors that failed `lint / build`:
unused Crossfade + GenreGridSkeleton imports, and unused
dMinScale / bbMajScale / Ab4 audio constants.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
