Slate v2 unwrapNodes should preserve moved child ids

What happened

Once exact-path wrapNodes(...) was real, the structural counterpart was unwrapNodes(...).

The tempting lie was to rebuild the unwrapped children as new nodes.

That would have destroyed the ids of the children that were merely moving up one level in the tree.

What fixed it

The honest exact-path cut unwraps by composition:

1. move each direct child out of the wrapper
2. remove the now-empty wrapper

That keeps the moved children as the same logical nodes:

• same runtime ids
• new sibling paths
• wrapper disappears

Reusable rule

For Slate v2 structural unwrap helpers:

• moved children keep their ids
• only the removed wrapper disappears
• do not rebuild descendants that are only moving structurally

If unwrapNodes(...) recreates the children, it is not unwrapping. It is replacement wearing a fake moustache.