master-part spec: state that a dependency on the master implies its parts #222

The book (30-design/053-master-part.ipynb) teaches that a dependency on a master is transitively a dependency on all of its parts — referencing the master guarantees every part row exists. This is a design consequence downstream tables rely on.

reference/specs/master-part.md covers structure, delete, and query semantics but does not state this transitive-completeness guarantee.

Proposal: add a subsection to reference/specs/master-part.md stating the guarantee, and a short note in how-to/master-part.ipynb.

From an internal datajoint-book → docs coverage review (priority P6).