improve get_neighbors performance

[lili2012]

I use graaf a lot and change get_neighbors my executable run time drop from 700s to 60s. Hope you could accept it. Thanks!

[github-actions]

Hi there! Thank you for creating your first pull-request on the Graaf library :)

[github-actions]

Marking this PR as stale. It will not be automatically closed.

Even though the maintainers of Graaf may not always have time to take a look in a timely fashion, your contributions are much appreciated.
Please allow some time for @bobluppes to take a closer look.

[github-actions]

Marking this PR as stale. It will not be automatically closed.

Even though the maintainers of Graaf may not always have time to take a look in a timely fashion, your contributions are much appreciated.
Please allow some time for @bobluppes to take a closer look.

[bobluppes]

Thanks for this — great catch, and a very reasonable optimization! Verified locally: builds cleanly against current main, all 752 existing tests pass (including the dedicated GetNeighbors coverage for both directed and undirected graphs), and clang-format clean. The reference-lifetime tradeoff (the returned reference is tied to the graph's internal state until that vertex is removed) matches the existing pattern already used by get_vertex()/get_edge() in this class, so this is consistent with the rest of the API. Merging, thanks again for contributing!