non final methods check

hey @JakeWharton first of all thanks for the great work.

I'd love to see _nopen_ also checking non-final methods when they should be.
I believe it makes sense to be within this lib, but this is you to say.

The idea is pretty much similar to @Open, but for methods.
A method should be final (when possible) or annotated.

I've made a Draft PR (more like a POC), #8 and happy to chat about.
If you are willing to bring this in and accept a PR, I'd gladly work on it.

Thanks a lot :)

Start by reviewing Draft PR #8 and the existing @Open checker, since the issue describes the proposed method check as analogous to it. Done means non-final methods are reported when they can be final unless they are appropriately annotated, with the behavior validated against the checker’s existing expectations.

Written by the indexing model from the issue text.