More accurate view distance based graph expansion

After moving each character, Graph::ensure_nearby is used to expand the graph to contain all nodes whose centers lie within the view distance, and all neighbors of those nodes. Because chunks cannot be populated with terrain until the cell of the cubic honeycomb containing them has all incident nodes generated, this leads to characters being within view distance of chunks that lie within generated nodes but which cannot be populated as they have incident nodes which have not yet been generated.

To correct this without expanding the graph at an unsustainable rate, we need to use a chunk-aware traversal that populates nodes if they are incident to a chunk which is within view distance. To avoid requiring a hyperbolic cube vs. sphere intersection test, we can approximate chunk visibility by considering node vertex visibility or using bounding spheres, as in #49.

Start at Graph::ensure_nearby and read issue #49 for the related visibility approach. The work is complete when traversal accounts for chunks within view distance, generates the incident nodes needed for population, and avoids unsustainable graph expansion.