Improve typeclass safety

[sg495]

Tasks:

• Define abstract classmethod Box._typeclasses(cls) -> frozenset[TypeClass] and implement it in FinRel.
• Implement final classmethod Box.typeclasses which calls _typeclasses on all final/leaf subclasses and takes the union (relies on Improve InheritanceForestMeta #14).
• Ensure that Box.spider is called on a compatible typeclass.