README demo recording

[yktsnd]

The README currently shows static screenshots of the real application. A short animated preview would communicate the product (particle flow, synchronized charts) much faster than stills.

Acceptance criteria

• The recording is captured from the actual running application (no mockups or staged frames).
• Playback, particle motion, and the synchronized charts are clearly visible in the clip.
• File size stays small enough for a README (target well under ~5 MB; short loop, cropped framing).
• The existing static hero screenshot is retained as a fallback for contexts where the animation does not load.
• Descriptive alternative text accompanies the recording.

Notes

A reproducible capture path (e.g. extending scripts/capture-screens.mjs with Playwright video + ffmpeg conversion) is preferred over a manual screen recording so the asset can be regenerated when the UI changes.