[codex] Add YouTube demo link to README - #4

Summary

• Adds the unlisted YouTube URL as the primary reviewer watch link for the point-by-point operator console demo.
• Keeps the existing poster and local MP4 fallback in the README.
• Preserves the no-hardware boundary: this is a real Tkinter UI replay of a deterministic 72-point simulated fixture, not live hardware validation, and production AWG/oscilloscope adapters are not used for the demo.

Validation

• git diff --check origin/main..HEAD
• Verified README references https://youtu.be/fYokRzNnm84
• Verified local fallback files exist:
  • docs/images/auto-load-off-test-point-replay-demo.mp4
  • docs/images/auto-load-off-test-point-replay-demo.png

Notes

This is a docs-only follow-up because the prior PR #3 is already merged.