Background program: alternate video and MicroW8 demos every 8 bars

The "video" background mode is now a PROGRAM: while live it cuts between the found-footage layer and a FRESH demo cart every 8 chord-bars (idle stays ambient video; the demoscene-only mode is unchanged).

Verified: explorer-ui H-series — 24 unit-driven ticks flip sides exactly 3x, the cart advances each demo turn, and the enabled layer tracks the active side.

▶ Play it: Mercury Interrogation — where the harmony organ has the least anywhere to hide https://stellate.app/?genre=bebop&seed=6976&path=694.13834,713.13867,675.13867&xdur=13.122

5888e2d https://github.com/aboard-io/stellate/commit/5888e2d8462539654c6293a4138090c301d4a75e