Add notebook kernel lease guard

[HunterCML]

/claim #12

Summary

• add dependency-free notebook kernel lease guard for the real-time collaborative editor
• validate kernel lease owners, expiry, handoff status, collaborator activity, resource limits, stale outputs, source hash drift, and unresolved cell comments
• emit execution queue, reviewer artifacts, SVG preview, and H.264 demo video

Validation

• node notebook-kernel-lease-guard/test.js
• node notebook-kernel-lease-guard/demo.js
• node --check notebook-kernel-lease-guard/index.js
• node --check notebook-kernel-lease-guard/test.js
• node --check notebook-kernel-lease-guard/demo.js
• ffprobe -v error -show_entries format=duration,size -show_entries stream=codec_name,width,height -of default=noprint_wrappers=1 notebook-kernel-lease-guard/demo.mp4
• git diff --check