This is Céu[0] erasure! ... I'm joking, I'm joking, although I do think it deserves a mention.
Seriously though, neat library! It took me a moment to realize that everything with a pa_ prefix is a macro, for the dumb reason of being used to only see those in ALL_CAPS. Not saying you can't use lower-case macros but I think a short sentence mentioning it before the demo code, and with a "see the protothread under the hood page[1] for an explanation of how it generally works" would help a lot with demystifying the code for people unfamiliar with the concepts involved.
https://github.com/frameworklabs/proto_activities