The only thing that tripped me up was:
The amount of space used DOES depend on the fragmentation of the index that supports the FOREIGN KEY constraint.
I took this to imply that such an index exists. "A-ha! There is no such an index!" is a bit sneaky but fair, I suppose. You did provide the entire table creation script so I can't complain.
ron
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a haiku...
NULL is not zero
NULL is not an empty string
NULL is the unknown