docs(readme): improve demo video preview

[jack-arturo]

Summary

• replace the README demo with a newly uploaded MP4 whose first frame is a nonblank title slate
• render the demo as one video element and move the fallback link to the PR asset comment so GitHub does not auto-render a duplicate video
• keep generated media/source out of repo history; the PR changes README.md only

Tests

• uv run --extra dev ruff check .
• uv run --extra dev ruff format --check .
• uv run --extra dev pytest -q
• verified video URL with ranged GET: 206 Partial Content, Content-Type: video/mp4, 3,722,212 bytes
• verified rendered GitHub README branch view has videoCount=1, one text fallback link, and the simulated-data disclaimer

Breaking Changes
None.

[jack-arturo]

Demo video asset: https://github.com/user-attachments/assets/c690d641-8f8d-4bbd-8462-3d2c2e25b38e

[Copilot]

Pull request overview

Replaces the Markdown link to the demo video with an inline HTML <video> embed plus a text fallback link, so the README renders the demo as a playable preview on GitHub while still providing a clickable fallback.

Changes:

• Add an HTML <video> element pointing to the existing user-attachments asset.
• Keep a <p><a> fallback link to the same asset for renderers that don't support <video>.

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