docs: refresh the v0.3.1 project demo

[Rcloudso]

Product impact

The repository landing page now demonstrates the current v0.3.1 reviewed OCR
knowledge workflow instead of foregrounding the v0.2.6 document-RAG demo.

Scope

• replace the bilingual README hero GIF and video links with v0.3.1 media
• show current Paddle/DeepSeek comparison, Block review, and grounded source evidence
• add the repeatable capture boundary to the v0.3.1 evidence package
• preserve the older v0.2.6 AI-assisted development case study as labeled historical material

The MP4 is already attached to the existing v0.3.1 Release, and its download
URL is live.

Verification

• focused Playwright v0.3.1 upload → OCR evidence → review → save v2 → publish → chat provenance walkthrough passed
• MP4: H.264, 1280×720, 17.92 s, 873,545 bytes; full decode passed
• GIF: 880×495, 18 s, 1,279,628 bytes; full decode passed
• GitHub asset SHA-256 matches the local MP4 and its download link passed
• git diff --check passed
• Standards review: 0 findings
• Spec review: 0 findings

Remaining risk

This is a deterministic browser capture using fixed project-contract OCR
responses. The release evidence separately records the successful real Paddle
worker and authenticated HTTP boundary; the demo does not claim a
private-document model benchmark.