Release v7.9.3

Changed

• Register derived types for non-system base classes in addition to the primary target type so polymorphic deserialization can resolve by base class without explicit targetType.
• Preserve existing validation semantics for ambiguous targets and target mismatches.

Added

• Serialization specs covering base-class derived type lookup and HasDerivatives behavior for known base types.
• Documentation clarifying backend target resolution and frontend runtime candidate resolution from both field derivatives and runtime derived-type registry.