feat: realtime collaboration demo

[Geliogabalus]

Summary

• Adds a new realtime-collaboration/ts/ demo built with Vite + TypeScript, JointJS Core, Yjs (CRDT), and y-webrtc (WebRTC P2P)
• Real-time synced diagram editing, cursor broadcasting, and selection locking across peers
• Collaborative user list with editable names, color-coded presence, and live editing indicators

Features

• Diagram sync — create, move, edit, and delete shapes/links; changes propagate instantly via Yjs Y.Map
• Cursor tracking — live cursor positions broadcast via awareness
• Selection locking — prevents two users from selecting the same cell simultaneously
• Cell editing lock — dashed highlight shown to remote peers when a cell is being edited
• User list — shows all connected users with color bullets; name is editable inline; highlights when a peer is renaming themselves; blocks canvas interactions while name is being edited