Slate v2 Op-family Fifth Slice

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

Goal

• Land the next honest core-family step in slate: exact split_node plus a narrow Transforms.splitNodes(...) helper.

Finding

• The docs already listed split_node and Transforms.splitNodes(...).
• The current core already had descendant splitting helpers, but no public split operation or helper.
• The honest first cut is text-node splitting, not broad legacy NodeOptions parity.

Slice

• Add real split_node support for text paths first, then exact path-based element splits.
• Add a narrow Transforms.splitNodes(...) helper:
  • { at?: Point } for text-node splits
  • { at: Path, position } for exact path-based element splits
• Keep left-branch runtime ids stable, allocate a fresh right-branch id, and rebase selection inward.

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