Called when the node's INodeView.Root is detached from its UI panel.
Fires whenever the visual element receives a DetachFromPanelEvent. Like NodeView<T0>.OnViewAttached, this can fire multiple times in a node's lifetime (tab switches, block-node drags, and so on), and a matching NodeView<T0>.OnViewAttached may follow. Use this callback to release resources tied to panel presence, such as event subscriptions or scheduled tasks.