The namedtuple-to-dataclass change made ForeignKey unhashable, breaking
set(table.foreign_keys) and dict-key usage that worked in 3.x. frozen=True
restores immutability and hashability. Equality and hashing cover all
compared fields including on_delete/on_update - two foreign keys differing
only in their actions are different constraints.
Refs https://github.com/simonw/sqlite-utils/issues/769#issuecomment-4900034150
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>