docs: link Proof Gate demo directly to MP4

[kirder24-code]

Summary

• Updates the README proof demo poster link to point directly at the raw MP4 on main.
• Adds a direct MP4 text link below the poster for GitHub users whose browser opens the file page instead of the video.

Scope

Docs-only follow-up. Only README.md changed.

Validation

• npm run check
• npm test
• npm run test:tier3
• git diff -- README.md
• git status -sb