Slate v2 merge_node should keep the left-branch id and rebase inward

What happened

Once exact split_node existed, the matching merge_node half of the pair was the next honest core-family step.

The tempting lie was to ship a broad mergeNodes(...) helper immediately.

That would have hidden the actual rule that matters:

• the left branch is the surviving logical node
• the right branch is the one being absorbed

What fixed it

The honest first cut stayed narrow:

1. add exact merge_node for text paths first, then exact path-based element merges
2. add a narrow Transforms.mergeNodes(editor, { at })
3. require matching text props on both sides
4. keep the left branch id
5. rebase selection inward onto the surviving left branch

Reusable rule

For Slate v2 merge semantics:

• keep the surviving left-branch id
• remove the absorbed right-branch id
• rebase selection inward onto the surviving branch
• do not claim broad mergeNodes(...) parity before the engine owns more than the exact merge case

If merge keeps the wrong id or hides branch ownership, downstream selector and range-ref reasoning rots fast.