networkx.Graph is too heavyweight for our segmentation

networkx.Graph makes copious use of Python dictionaries, which are of course memory hogs. It would dramatically improve space efficiency to replace it with a different graph class that had the same interface.

Start by locating where networkx.Graph is used for segmentation and identify the interface that must remain compatible. Compare the current graph's memory use with a replacement design, then verify that segmentation behavior is unchanged; the issue does not name files or tests for validation.

Written by the indexing model from the issue text.