Hierarchical knowledge graph for the Map screen.

Modes (responses stay {nodes, edges}; hierarchical modes add a sibling meta: {total, truncated} which clients may ignore): · default — overview: 100% of the corpus aggregated into hubs · ?topic=X — one topic's members (X=untopic → no-topic rows) · ?category=X&tail=1 — a category rollup's tail topics as sub-hubs · ?docs=1 — the document hub's rows