Slate v2 Op-family Fourth Slice

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

Goal

• Land the next narrow text-op follow-on in slate: exact remove_text plus Transforms.removeText(...).

Finding

• The current docs already listed remove_text in the operation taxonomy.
• The package still lacked the operation and an exact text-removal helper.
• The honest helper is removeText(...), not broad legacy Transforms.delete(...) parity.

Slice

• Add real remove_text support in the core transaction engine.
• Add exact Transforms.removeText(editor, text, { at? }).
• Rebase selection inward on text removal.
• Keep the slice narrow:
  • exact text removal only
  • no broad delete(...) semantics
  • no unit/distance/hanging parity

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