The kinds this store writes. Every released reader of relations.json
deserializes the kind strictly, so a new kind is added only once the
oldest supported reader parses it; kinds a newer writer stored are kept
as written (see KnowledgeRelationGraph::load).
Average relations per current fact the graph may hold before its weakest
edges are pruned. Knowledge has no fact limit; this bounds edge density
(an edge-producing loop cannot grow the graph faster than the facts it
links), so the bound grows with the store instead of capping it.