Support class relationships and foreign keys

`datafiles` package is great, but it stops short of providing support for object relationships and foreign keys. is that feature anywhere on the horizon? if not, would you be open to discussing it and providing guidance on how to implement it?

No files, tests, or entry points are named. Start by reviewing the datafiles ORM’s dataclass and serialization design, then clarify the expected relationship and foreign-key behavior; done requires an agreed scope and corresponding implementation guidance.

Written by the indexing model from the issue text.