What guarantees does Pkg provide if I do Ctrl+c while it’s doing something?
None. Cross your fingers.
But seriously, it makes an effort to do things atomically so that things don’t get messed up but a lot of people have worked on the code and have taken different degrees of care about atomic file system operations (which are not easy). Guarantee is a strong word.
FWIW, I Ctr-C stuff all the time when using Pkg and I still haven’t gotten in a bad state.