MicroW8 demoscene background layer

A self-contained DemoLayer module runs vendored MicroW8 (public-domain) carts on a canvas behind the star map, screen-blended with the found-video so both show through.

• Reacts to the music: a bar-level pulse plus per-note reactivity (DemoLayer.note) fed from the explorer's timeline — bass swells the clock, leads flash and hue- shift by pitch class, drums tick

Verified: demo-layer-test (32 carts animate, note-perturbation, cycling).

▶ Play it: Seismic Ultimatum — the genre that is mostly bass https://stellate.app/?genre=dubstep&seed=8147&path=1379.18226,1398.18259,1360.18259&xdur=13.122

fd72345 https://github.com/aboard-io/stellate/commit/fd723453c2e4eb75afc093b5e18c13ad71ce6081