Links AttributeSets to external type classifications.

Allows the host app to classify attribute sets by purpose: "CMS block definition", "entity schema", "piece", "gallery", etc.

The type_id references whatever taxonomy/type system the host app uses. AttrEngine doesn't own the Type schema — it just stores the foreign key.

Primary flag

Each AttributeSet can have one type marked as primary: true to identify its main classification when it belongs to multiple types.