Add music visualizer example

Should load a song as a static resource, and play it with some kind of visuals effect. Possibly just a grid of four cells that change color according to keyboard events and frequency bands. Or perhaps a spectrogram would be cool.

The issue names no files, tests, or entry points. Begin by surveying the repository's existing examples and determine whether the visualizer should use keyboard-driven cells or a spectrogram; done should be a runnable music-visualizer example that loads a static song and displays responsive visuals.

Written by the indexing model from the issue text.