GenerativeUiInterfaceCloseSurfaceMessage

Remove the surface and return the device to its home screen. Call it when the UI is no longer needed; a surface stays held until it is closed or replaced, even if the viewer leaves it with the remote. Completion: accepted means the gateway took the message, not that the surface is gone. It takes the next sequence of the surface, so it applies after every update accepted before it. Refused with a conflict when `surfaceId` is not the surface Port OS holds for the device. Confirm with get-surface, which then reports no surface.