[snchimata]

Summary

• Pin the README demo URL to the commit containing the refreshed GIF instead of the cacheable main-branch URL.
• Add an offline regression check requiring an immutable demo URL.

Verification

• Reproduced the live GitHub page loading the old 900x400 image while the repository and direct origin have the new 1280x720 asset.
• Verified the pinned URL decodes to 1280x720 in that same browser.
• New regression failed before the URL change, then passed.
• 24 focused Python tests passed with 47 subtests; git diff --check passed.

No GIF regeneration, SDK changes, or release publication.