Gets runtime-registered derived type mappings, keyed by polymorphic base type.
public IReadOnlyDictionary<Type, IReadOnlyList<TomlDerivedType>> DerivedTypeMappings { get; init; }
These mappings are used by reflection-based serialization and are merged with attribute-based registrations. Base-type attributes take precedence when the same derived type or discriminator is registered in both places.