In the master branch, Vector::UNIT is defined as the zero-vector (0, 0),
which is obviously not correct. But I’m unsure, what value would be correct.

According to this, a unit vector is any vector with length 1, so there would be many valid values. In terms of what constants would be useful I would suggest providing the unit vectors of the standard basis:

What’s your interest in this particular piece? To me it looks like a bad copy-paste for a constant that is not even being used yet. It will probably be fixed when the author starts using it.