ForeignKey is now a frozen dataclass, restoring hashability

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>

docs/changelog.rst

@@ -19,6 +19,7 @@ Unreleased
 - Fixed a bug where a failed write statement executed with ``db.execute()`` left the driver's implicit transaction open. Every subsequent write then joined that phantom transaction, which nothing committed, so their work was silently rolled back when the connection was closed. The implicit transaction opened by a failed statement is now rolled back before the exception is raised. A failed write inside a transaction opened with ``db.begin()`` or ``db.atomic()`` leaves that transaction open and untouched, as before.
 - Fixed a bug where transaction-control statements prefixed with an empty statement - ``db.query("; COMMIT")`` - or a UTF-8 byte order mark slipped past the check that rejects them, committing the caller's open transaction before raising a confusing ``OperationalError``. The keyword scanner used by ``db.query()`` and ``db.execute()`` now skips leading ``;`` and byte order marks, matching what the ``sqlite3`` driver tolerates before the first token, so these statements are rejected with a ``ValueError`` without being executed. The same fix means ``db.execute("; BEGIN")`` no longer auto-commits the transaction it just opened.
 - Documented a limitation of ``db.query()``: a ``PRAGMA`` statement that returns no rows raises a ``ValueError`` but still takes effect, because PRAGMA statements run outside the savepoint guard used to roll back other rejected statements. Use ``db.execute()`` for row-less PRAGMA statements.
+- ``ForeignKey`` objects are hashable again. The 4.0 change from ``namedtuple`` to dataclass accidentally made them unhashable, breaking patterns like ``set(table.foreign_keys)`` that worked in 3.x. ``ForeignKey`` is now a frozen dataclass - immutable and hashable, like the namedtuple was.
 - Fixed a bug where compound primary key columns were returned in table column order instead of ``PRIMARY KEY`` declaration order. For a table declared as ``CREATE TABLE other (b TEXT, a TEXT, PRIMARY KEY (a, b))`` an implicit ``FOREIGN KEY (x, y) REFERENCES other`` was introspected as referencing ``(b, a)`` when SQLite resolves it as ``(a, b)`` - running ``transform()`` on such a table then rewrote the schema with the inverted column order, silently reversing the meaning of the constraint and causing foreign key errors on valid data. ``table.pks``, compound foreign key guessing and ``transform()`` now all use the primary key declaration order, and ``transform()`` no longer reorders a compound ``PRIMARY KEY (b, a)`` into table column order.
 
 .. _v4_0rc3:

docs/upgrading.rst

@@ -95,7 +95,7 @@ Python API changes
     for fk in db["courses"].foreign_keys:
         fk.table, fk.column, fk.other_table, fk.other_column
 
-Attempting the old unpacking or ``fk[0]`` indexing now raises ``TypeError``, so any code using those patterns will fail loudly rather than silently misbehave.
+Attempting the old unpacking or ``fk[0]`` indexing now raises ``TypeError``, so any code using those patterns will fail loudly rather than silently misbehave. Like the old namedtuple, ``ForeignKey`` instances are immutable and hashable - they can be collected into sets and used as dictionary keys. Note that equality now includes the ``on_delete`` and ``on_update`` actions: a ``ForeignKey`` with ``ON DELETE CASCADE`` is not equal to one without.
 
 Compound foreign keys - previously returned as one ``ForeignKey`` per column, misleadingly suggesting several independent single-column keys - are now returned as a single ``ForeignKey`` with ``is_compound=True``. For these the scalar ``column`` and ``other_column`` fields are ``None``; use the ``columns`` and ``other_columns`` tuples instead. Single-column foreign keys are unaffected apart from the class change: ``column``/``other_column`` behave as before and ``columns``/``other_columns`` are one-item tuples.