IO processor causes motor spool-up on aircraft with reverse thrust

## Bug report

**Issue details**

In situations where the IO controller is no longer communicating with the FMU (soft reboot, firmware update, etc.) some situations seem to exist where the IO controller asserts low PWM on the throttle output channels(s). In most situations this would not cause issues. However, it can result in unintended spool-ups to full power in one direction on aircraft configured for reverse thrust.

**Version**
Plane 3.9

**Platform**
[ ] All
[ ] AntennaTracker
[ ] Copter
[x] Plane
[ ] Rover
[ ] Submarine

**Airframe type**
Hitec Xeno FX

**Hardware type**
Cube Black

**Logs**
https://youtu.be/ZBMU6mb1TIQ

Start by reviewing the IO controller and FMU communication behavior described in this report, including the linked reproduction video and the Plane 3.9 context. Trace how low PWM is handled during communication loss and verify that reverse-thrust aircraft cannot spool up unintentionally; the issue is done when this failure mode is reproduced, corrected, and covered by an appropriate test.

Written by the indexing model from the issue text.