Function render_calib 

async fn render_calib(
    driver: &mut Driver<SpiTransport<Spi<'static, Async, Master>, Output<'static>, Output<'static>, 2>, 2>,
    state: BacklightState,
) -> Result<(), SpiTransportError<<Spi<'static, Async, Master> as ErrorType>::Error>>

Render the full-travel calibration frame.

The background gradient starts red at 0 % progress and shifts toward blue at 100 % via state.calib_pct. Every LED whose bit is set in state.calib_leds_done is overlaid with solid green regardless of the background, giving immediate per-key visual confirmation as each key is pressed to the bottom.

Errors

Returns Err if any bus transaction fails. See [BacklightDriver::flush].