A small batch, written now, answered with the rows.
The nested path is the exception annex 05 §1.2 allows and the reason is the same one it
gives: the base is not a filter here, it is the thing being written into. Reading items
stays flat at /knowledge-items?kb_id=…, because three of the four placements have no
kb_id to nest under.
The body shows the fields the schema always carries. Which combination is legal is a service rule the schema cannot express, so read the description above before copying this.