feat(vector-search): make secondary sort stable within distance buckets

[nkneupper]

When using _vector_distance(buckets: N) or _vector_distance(bucket_size: N), results within the same bucket were being reordered by document key as a tiebreaker, disrupting the original vector distance ordering when secondary sort values were equal. Add is_greater_kv_group_stable (without the key tiebreaker) so that std::stable_sort preserves the pre-bucket ordering when scores are tied.

My product use case for this stable sort is to utilize the secondary sort as a kind of "search boost". The vector distance is the primary sort, but within each bucket, a secondary sort can apply to locally boost certain results within that bucket.

I am open to feedback and would be willing to explore an interface change to optionally make this secondary sort stable rather than having it stable by default (e.g. sort_by: _vector_distance(bucket_size: 25):asc, boost_score:asc:stable)

Change Summary

• Added KV::is_greater_kv_group_stable for stable sorting
• Added stable sort test to collection_sorting_test.cpp

PR Checklist

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