OptionaldocDocuments under the topic, when tracked.
Topic nodeId (matches topicPath entries). Churns on rebuild — not a persistable handle.
Hierarchy level: 0 = leaf tier, higher = coarser tiers.
Topic display name. Re-resolves by name across rebuilds.
OptionalparentParent topic nodeId, when the topic sits under a coarser tier.
A topic (theme) node of the graph's concept layer, as referenced by KnowledgeBaseGraphNode.topicPath.