Align abstract additionalProperties rules with the Core meta-schema

[clemensv]

Problem

Core says that abstract types implicitly permit additional properties and that additionalProperties MUST NOT be used on abstract types. The Core meta-schema nevertheless permits an abstract object schema to declare additionalProperties.

As a result, SDK schema validators can accept schemas that contradict the normative prose. This was encountered while analyzing json-structure/sdk#184, where abstract bases declaring additionalProperties: false passed schema checking.

Required resolution

Choose and consistently specify one model:

1. Keep the current prose: abstract object and tuple schemas always remain open, and declaring additionalProperties on them is invalid; or
2. Permit additionalProperties on abstract types and define precisely how it composes into extending concrete types.

The first model matches the current normative text and avoids a closed abstract base preventing derived properties.

Acceptance criteria

• Normative prose, Core meta-schema, and examples agree.
• The meta-schema rejects prohibited combinations where possible.
• The interaction between an open abstract base and a closed concrete derived object is demonstrated.
• SDK conformance tests can distinguish valid and invalid abstract schemas without implementation-specific assumptions.