Trim demo opening and add looping preview

[tmustier]

Summary

• start the full-resolution demo on Pi's populated startup screen instead of a blank frame
• add a lightweight, continuously looping 2× GIF preview to the README
• retain the high-resolution queue screenshot and full MP4 link

Proof

• MP4: H.264, 1290×700, 30 fps, YUV420p; first frame is populated
• GIF: 960×521, 12 fps, 941 KiB, loops indefinitely; first frame is populated
• npm run ci — 30 tests pass
• git diff --check
• npm pack --dry-run includes GIF, MP4, and screenshot