Private/Protected Objects?

We should consider whether inheritance needs the notion of a private object. Marking an object as private would make it illegal to resolve with an override.

Protected would be the same but would allow overrides in sub-blocks.

No files or tests are named. Begin by tracing inheritance, override resolution, and sub-block handling in the repository, then clarify the intended private and protected semantics. Done means the design is settled and the resulting behavior is covered for both prohibited and permitted overrides.

Written by the indexing model from the issue text.