docs: propose moving selected nodes between Spaces

[Yuqing (mydmdm)]

Summary

• define the Move selected nodes and Frames between Spaces #142 user flow for moving selected nodes and Frame subtrees between Spaces
• preserve hierarchy, geometry, internal edges, styles, and required assets while reporting boundary-edge omissions and collisions
• bound reliability to determinate failures under a continuing Server process, using dual locking and compensation
• explicitly defer general multi-Space transactions, WAL/crash recovery, Blob GC, and multi-backend transaction design

Scope

This PR contains the proposal only. Implementation will follow after design review.

Validation

• pnpm typecheck
• pnpm format
• pnpm lint:fix

Closes #142