Gets runtime-registered derived type mappings, keyed by base type.
public IDictionary<Type, IList<YamlDerivedType>> DerivedTypeMappings { get; }
Entries registered here are merged with attribute-based registrations (YamlDerivedTypeAttribute and ). Attribute-based entries take precedence when the same discriminator or type is registered in both.
This enables cross-project polymorphism where the base type and derived types live in different assemblies (e.g., clean architecture, plugin systems).