Support DJI style arm/disarm gestures

For those users who are transferring from DJI (or flying both DJI and AP vehicles) it is convenient if they can continue to use the same arm/disarm stick gestures as DJI. We probably only need this on Copters although it would do no harm on other vehicles.

The code implementing this is [here in motors.cpp](https://github.com/ArduPilot/ardupilot/blob/master/ArduCopter/motors.cpp#L29)

This was received at the Japan Drone Expo 2025

Start by reading ArduCopter/motors.cpp at the linked section to understand the existing arm/disarm gesture handling. Confirm the intended DJI stick gestures and their safety implications, then determine how Copter should recognize them without affecting other vehicle types; done means the gestures work as documented and existing arming behavior remains safe.

Written by the indexing model from the issue text.