docs(readme): embed demo videos and move experimental AI sections

[chaxus]

Merge Verdict

[APPROVE] — Documentation changes only; no application or deployment changes.

Summary

Repository MP4 links opened file pages instead of a playable README video. Replace them with four GitHub video attachments in the English and Chinese READMEs and the demo guide. Move the assistant and WebMCP sections after Development in all eight READMEs and label the assistant Experimental Beta. Keep the original MP4 downloads, caption files and model limitations.

Risk Analysis

Low: GitHub provides the attachment player. Separate WebVTT captions remain linked in the demo guide; the GitHub player does not attach those caption files automatically.

Design Decisions

Use standalone attachment URLs supported by GitHub Markdown. No new website, player or hosting configuration.

Code Concerns

No blocking findings in the documentation diff or affected references.

Verification

• 38 README locale compatibility tests passed.
• Prettier and git diff --check passed.
• All four GitHub Markdown video players were exercised with Play. Public byte-range GETs returned 206 Partial Content with video/mp4 for every attachment.
• Application E2E suites were not rerun locally for this documentation-only change.

Demo attachments

Word

word-ai.mp4

Excel

excel-ai.mp4

PowerPoint

pptx-ai.mp4

PDF

pdf-ai.mp4

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