Node tree: Selecting specific nodes instead of the root tree

Currently, the node tree only provides "int isSelected" instead of "int selectedId".
This means that the entire tree is selected instead of a child node.

Start at the node tree API and trace how the current int isSelected value is used during selection. Confirm how a child node can be identified, then make the node tree expose a selected ID so selection targets that child rather than the root tree; the issue names no file or test to run.

Written by the indexing model from the issue text.