Measure whether graph is strongly connected

This issue captures a TODO item in the code:

https://github.com/apache/lucene/blob/4e3945ed548f40f31498dddcdce611c5cbbec245/lucene/core/src/java/org/apache/lucene/util/hnsw/HnswUtil.java#L220-L221

Start with lucene/core/src/java/org/apache/lucene/util/hnsw/HnswUtil.java at lines 220-221, then read the surrounding HNSW utility code to understand the TODO and how graph connectivity is represented. Identify the existing entry point for measuring connectivity and verify the result on representative graphs; done means the TODO is implemented and the strongly connected case is covered.

Written by the indexing model from the issue text.