I’m sorry, I must have missed which article and section of the Constitution defines citizenship. Please cite the specific section of the Constitution were citizenship is defined.
When I said "lays out the rules for" I meant rules for who is a citizen by birth, which of course is done in the 14th. I'm not aware of any definition of what citizenship "is", either in the Constitution or in statute.
I'll grant that Congress can and has also granted birthright citizenship in certain cases, but that's not the same thing as naturalization.