Slate v2 Op-family Eighth Slice

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

Goal

• Land the next narrow selection helper in slate: Transforms.collapse(...).

Finding

• The current docs already described collapse(...).
• The package only had select(...), setSelection(...), and deselect(...).
• The important seam was not the API name. It was reading the live draft selection inside outer transactions.

Slice

• Add Transforms.collapse(editor, { edge? }).
• Support anchor, focus, start, and end.
• Keep it narrow:
  • no move(...)
  • no setPoint(...)
  • no broader selection-motion family

Verification

• yarn mocha --require ./config/babel/register.cjs ./packages/slate/test/snapshot-contract.ts
• yarn test:mocha
• yarn workspace slate-react run test
• 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