Called when the node's INodeView.Root is attached to a UI panel.
Fires whenever the visual element receives an AttachToPanelEvent. This can happen multiple times during a node's lifetime — for example when the user tabs away from and back to the graph view, or when a block node is dragged and re-parented under a different context. Use this callback for logic that needs to respond to panel-attach events, such as subscribing to panel-level events.