I'm kind of just using words. You can call it whatever you want, I just used words that my friends also used to describe what I wrote. Really, loading custom libraries from C is really easy but I decided to advertise that part of the project anyway.

I have a related question: is there a different terminology for when you can call an arbitrary C function versus when you need to implement a C function with a special signature (which often can be used to bridge the target C function) ?

If you have to implement C code and load it, you're making "extensions", "plugins", "(low-level) modules", ... it is not FFI.

Nothing.