Slate v2 Op-family Seventh Slice

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

Goal

• Land the next narrow selection-transform follow-on in slate: Transforms.setSelection(...) and Transforms.deselect(...).

Finding

• The current docs already described both helpers.
• The package only had raw select(...).
• The subtle bug was helper semantics inside outer transactions: reading editor.selection would have used stale committed state instead of the live draft selection.

Slice

• Add Transforms.setSelection(editor, props).
• Add Transforms.deselect(editor).
• Keep the slice narrow:
  • no collapse, move, or setPoint yet
  • no broad legacy selection-transform parity
  • live draft selection is the important seam

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