Attach a compressed demo walkthrough to the README

[AlaskanTuna]

A recorded walkthrough exists but has not been compressed or attached anywhere. Because the app is down, the video is currently the only artefact that demonstrates the product — which makes it the one thing that decouples the LinkedIn post from the migration timeline. Attaching it to the README (and to the GitHub release/social preview) means the repo tells its story even while solarsim.tech is dead.

Scope: README.md, docs/ (link only — the binary itself is uploaded to GitHub, not committed)

Checklist

• Compress the recording to a web-friendly target (H.264 MP4, ≤ 1080p, CRF ~26, audio dropped or ≤ 96 kbps) and confirm the result is under GitHub's per-file upload ceiling for issue/release attachments
• Upload as an attachment to a GitHub release or an issue comment and copy the resulting CDN URL — do not commit the binary to the repo
• Embed in README.md above the feature list, with a one-line caption naming the three-page flow (Map → Workbench → Analysis)
• Add a plain-text note beside the embed stating that the hosted demo is temporarily offline pending a database migration, linking to the v1.1 — Back Online milestone — honesty here reads better than a dead link
• Set the repo social preview image to a still frame from the video (Settings → Social preview) so the LinkedIn link unfurls with a real screenshot
• verify: the README video renders and plays on github.com in an anonymous browser session
• verify: pasting the repo URL into a link-preview checker produces the intended social card
• verify: git status shows no video binary staged, and the repo clone size is unchanged

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_{Priority: High · Phase 12 task 3 of the maintainer's migration plan.}