moveChild

Detaches child from its current parent, if any, and appends it to this node, preserving both size dimensions on both parents. Used by merge to relocate tombstoned children as RGA anchors without corrupting index positions.

Differs from detachChild followed by append: a removed node contributes no visibleSize to either parent (removal already excluded it from its ancestors), so only the include-removed totalSize is relocated for a tombstone. Reusing detachChild+append would double-subtract the tombstone's visible dimension.