@hackage hgg-custom0.2.0.0

An add-on package that isolates non-standard "custom marks" — new plot types built on hgg-core's customMark extension point — from the core library. It never touches core's MarkKind; it depends only on hgg-core's Primitive / customMark API. Add it as a dependency only when you need these specialized visualizations (e.g. for random forests or hierarchical clustering).

Currently ships a dendrogram mark (tree diagrams for hierarchical clustering), using pre-baked segments for easy cross-backend parity.