Explore sending output over GPIO about the state of VxScan to support hardware testing

[arsalansufi]

The HW team's paper feeder robot currently uses two external sensors, one at the infeed and one in the ballot box, to identify when paper is being scanned and when that paper has cleared the scanner. There's a timeout that if hit, strongly suggests that something has gone wrong (paper never cleared the scanner because of a jam, scanner crash, etc.).

We want to be able to log what actually went wrong (jam, scanner crash, etc.). The app emits its own logs, and we can use those. We'll just have to do some manual cross referencing of paper feeder logs and app logs, e.g., by timestamp.

If we were to emit error information from the app over GPIO (or some other protocol), the paper feeder robot could log this information itself. Definitely more convenient. And in general, getting direct signal from the app about the paper status will be more robust than inferring paper position using external sensors.

We should explore this when we build the VxScan GPIO integration for the VVSG alarm mechanism, but don't necessarily need to do this before then.