CheckIndex could assert more things about the HNSW graph?

### Description

I came across a spooky comment `// we see repeated neighbor nodes?!` in [this `luceneutil` PR](https://github.com/mikemccand/luceneutil/pull/290/files) and discussed with @msokolov and we realized `CheckIndex` tool could assert things about the HNSW graph like "outgoing neighbor nodes are in order" and "there are no duplicate neighbor nodes".

Maybe there are other "HNSW graph integrity" checks that `CheckIndex` could do?

Start by reading CheckIndex's existing HNSW validation and the linked luceneutil PR, especially the comment about repeated neighbor nodes. Determine which graph-integrity invariants are appropriate beyond neighbor ordering and duplicate detection; done means the scope is agreed and CheckIndex reliably reports violations with coverage for the added checks.

Written by the indexing model from the issue text.