More accurate view distance culling

Currently, Graph::nearby_nodes only considers node centers. This causes chunks which fall within view distance, but which lie in nodes that do not, not to be rendered. While an exact solution requires potentially nontrivial hyperbolic geometry predicates, we could achieve a significantly closer approximation by checking distance to each vertex of the node and rendering chunks which lie on vertices that fall within view distance even if the center of their node does not. Alternatively, chunk bounding spheres could be used as discussed in https://github.com/Ralith/hypermine/issues/100.

Start at Graph::nearby_nodes and review the issue 100 discussion about chunk bounding spheres. Compare vertex-distance and bounding-sphere approaches, then verify that chunks within view distance are rendered even when their node centers are outside it; the issue does not name a test or file path.