Latest electrical telemetry, split out of BB.TUI.State.

battery holds the most recent BB.Message.Sensor.BatteryState payload and power the most recent BB.Message.Sensor.PowerState, or nil until one arrives on the [:sensor | _] path. The status bar reads these to render an at-a-glance charge / voltage segment — most useful when driving a headless robot over SSH, where the charge level is otherwise invisible.

Only the freshest reading of each kind is kept; the event log carries the history.