Serializer: store classes as references, add aliases and schema versions

[0x1r1s]

Summary

The serializer stores a class as a reference (its module path), never as code. This PR extends that approach so that SerializerMixin can save types and sets, and so that saved data stays readable after the code changes.

1. Types, sets and keys. A class value is now saved as {'__type__': 'module.Class'} and resolved through the allowlist. Before this change, a class value became None with no error. set and frozenset are saved in sorted order, so equal sets give equal bytes. A class, an enum or a tuple can now be a dict key. Before this change, msgpack refused these keys.
2. Moved classes. register_class_alias(old_path, new_path) loads old payloads after a class moves or is renamed. Aliases can form a chain. A cycle is refused. The allowlist check applies to the new path.
3. Changed attributes. A class can set _schema_version and override _migrate(data, version). A load calls _migrate when the payload is older than the class. A payload that is newer than the class raises SchemaVersionError. Version 0 is not written, so the payloads of existing classes do not change.
4. Deleted classes. A missing module or class raises MissingClassError (a ValueError). The message names the path and refers to register_class_alias.

PropagationMixin no longer calls _set_parent on a class value after a load.

Backward compatibility

• I loaded each of the 73 objects in the OpenCell database and saved it again, with dev and with this branch. The bytes are the same for all 73. Each object loads again.
• The new error classes are subclasses of ValueError, so code that catches ValueError around deserialize continues to work.

Tests

• test/test_serializer_mixin.py: 28 new tests for type references, sets, aliases, missing classes and schema versions. The full steer-core suite passes.
• The full steer-opencell-design suite passes with this branch.

Next step

steer-opencell-production uses these features to save machines. After this PR is merged and released, that package must raise its steer-core version bound.

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