Slate v2 Op-family Third Slice

Supporting plan. For current queue and roadmap truth, see master-roadmap.md.

Goal

• Land the next narrow slate op-family follow-on: path-based Transforms.unsetNodes(...).

Finding

• The new set_node seam made path-based property updates real.
• The next honest follow-on was property removal through that same seam, not a wider new node-family jump.

Slice

• Add Transforms.unsetNodes(editor, props, { at }).
• Reuse set_node with the properties/newProperties pair for property removal.
• Keep it narrow:
  • path-based only
  • custom prop removal only
  • no broad legacy NodeOptions parity

Verification

• yarn mocha --require ./config/babel/register.cjs ./packages/slate/test/snapshot-contract.ts
• yarn test:mocha
• targeted tsc diagnostics on changed packages/slate files: 0 errors
• targeted yarn exec eslint on changed packages/slate files
• yarn prettier --check on changed slate-v2 files
• pnpm exec prettier --check on changed plate-2 docs