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

CHANGELOG.md

@@ -5,6 +5,30 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [0.2.25] - 2026-09-29
+
+### Added
+- **`SerializerMixin` stores classes as references.** A class value is saved
+  as `{'__type__': 'module.Class'}` and resolved through the allowlist;
+  previously it became `None` with no error. A class, an enum or a tuple can
+  now be a dict key.
+- `frozenset` round-trips (`__frozenset__`). Set items are now written in
+  sorted order, so equal sets give equal bytes. The 0.2.24 `__set__` payload
+  shape is unchanged.
+- `register_class_alias(old_path, new_path)` loads old payloads after a class
+  moves or is renamed. Aliases may chain; cycles are refused; the allowlist
+  check applies to the new path.
+- `_schema_version` / `_migrate(data, version)` on serializable classes. A
+  payload older than the class is migrated on load; a newer one raises
+  `SchemaVersionError`. Version 0 is not written, so existing payloads are
+  byte-identical.
+- A missing module or class raises `MissingClassError` naming the path.
+  Both new errors subclass `ValueError`.
+
+### Changed
+- `PropagationMixin` no longer calls `_set_parent` on a class value after a
+  load.
+
 ## [0.2.24] - 2026-09-18
 
 ### Added

steer_core/Mixins/Propagation.py

@@ -82,6 +82,15 @@ def wrapper(self, new_value):
     return decorator
 
 
+def _has_parent(value: Any) -> bool:
+    """Return True if ``value`` is an instance that can take a parent reference.
+
+    A class stored as a value (a type reference) also has ``_set_parent``, as an
+    unbound function. It must not be called.
+    """
+    return hasattr(value, '_set_parent') and not isinstance(value, type)
+
+
 class PropagationMixin:
     """
     Mixin providing update propagation through a hierarchical object tree.
@@ -320,21 +329,21 @@ def _set_parent_on_value(self, value: Any, attr_name: Optional[str] = None) -> N
             attr_name: The attribute name on this object that holds the value.
                 Used for setter-based propagation.
         """
-        if hasattr(value, '_set_parent'):
+        if _has_parent(value):
             value._set_parent(self, attr_name)
-        elif isinstance(value, (list, tuple)):
+        elif isinstance(value, (list, tuple, set, frozenset)):
             # For collections, items don't have a direct setter path
             for item in value:
-                if hasattr(item, '_set_parent'):
+                if _has_parent(item):
                     item._set_parent(self, attr_name)
         elif isinstance(value, dict):
             # Check both keys AND values - materials can be dict keys
             # For dict items, they don't have a direct setter path
             for item in value.keys():
-                if hasattr(item, '_set_parent'):
+                if _has_parent(item):
                     item._set_parent(self, attr_name)
             for item in value.values():
-                if hasattr(item, '_set_parent'):
+                if _has_parent(item):
                     item._set_parent(self, attr_name)
 
     def __deepcopy__(self, memo):