docs: refresh the hosted walkthrough video

[unlockdigital]

Summary

Refresh the hosted walkthrough with the approved 50.1-second cut: smoother pacing and cursor movement, unobstructed captions after transcript seeking, clearer citation timing, and a readable closing card with the homepage’s animated Loci mark.

Update the README video link, MP4, poster, animated preview, chapter timings, and media provenance notes together. The README opens the approved repository video through a clickable poster. This replaces the previous inline GitHub attachment, whose separate hosted URL would otherwise continue playing the old cut. Maintainers can optionally attach the new MP4 through GitHub to restore inline playback.

Merge order

Stacked on #11 to retain its validated dependency and reliability fixes. Please merge #11 first. The video-specific commit is ea28d59; its scope is six README/media files. Existing media permissions and attribution remain documented. Source interviews, collection datasets, and source 3D models remain outside this change.

Validation

• Approved v4 video and committed MP4 have identical SHA-256 hashes.
• 50.1 seconds, 1920×1080, 30 fps, H.264 video and AAC audio; full video decode passed.
• Public fixture guard and git diff --check passed.
• Poster, animated preview, and closing frames reviewed locally.

No application configuration or data migration is required.