Heh, I’m probably misusing that term—forgive me, I’m just a script kiddy! :D

I meant that it can serve the role of low-level, nitty gritty machinery, entirely separate from its use case as a module unto its own right.

Eg, pandas (in some sense) is just a convenience layer on top of numpy—but to me that’s like saying any piece of software is “just a convenience layer on top of python.” It’s partly a means to an end! Not just an end unto itself.

Numpy enables a whole new class of functionality, independent of its direct use as part of my software. Is there a better word for that?

I think you sum it up nicely! The better word you're looking for might be cornerstone?

perhaps foundation, or base layer.

Sidebar: I originally used the word “gunmetal” and that...was even less correct.

Get this guy a dictionary!