Slate v2 Op-family Sixth Slice

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

Goal

• Land the next matching core-family step in slate: exact merge_node plus a narrow Transforms.mergeNodes(...) helper.

Finding

• split_node was now real, but the matching merge_node half of the pair was still missing from the package.
• The honest first helper is path-based text-node merging, not broad legacy mergeNodes parity.

Slice

• Add real merge_node support for text paths first, then exact path-based element merges.
• Add a narrow Transforms.mergeNodes(editor, { at }).
• Keep left-branch runtime ids stable 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