inertiallabs/ardupilot ? reverse-engineered prompt

I want to get this ArduPilot codebase building cleanly on my machine and have a simple way to run the main vehicle simulators and tests. Please set it up so I can compile the flight software for Copter, Plane, Rover, and Sub, then run the built in test suite and fix anything obvious that breaks in a fresh checkout.

If anything depends on current setup steps or docs, look up the latest instructions online and follow them. Please also make sure the repo has clear, easy to follow build and run notes for someone who is not deep into autopilot development, including the basic commands to build, test, and start a simulation.

Keep the changes minimal and practical, focus on getting the project usable, and explain what you changed at the end so I know how to build and verify it myself.