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Reject early-abandoned distances with a strict comparison in MESSI top-k - #77

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fix/messi-topk-duplicate-results
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sophisid wants to merge 2 commits into
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fix/messi-topk-duplicate-results

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ts_euclidean_distance_SIMD stops once its partial sum reaches the bound and returns that partial sum. The top-k scans accepted results with dist <= knn[k-1], so a series whose abandoned partial sum equalled the current bound was inserted with that under-estimated distance, and its position was then reported next to a distance that belongs to another series. With the position-based de-duplication this wrong entry could also block the real series, because it already occupied the same position slot.

Compare with < in the approximate scan and in the MESSI L2 leaf scan, so a distance that is not strictly better than the current k-th is never inserted.

ts_euclidean_distance_SIMD stops once its partial sum reaches the bound and
returns that partial sum. The top-k scans accepted results with
`dist <= knn[k-1]`, so a series whose abandoned partial sum equalled the current
bound was inserted with that under-estimated distance, and its position was
then reported next to a distance that belongs to another series. With the
position-based de-duplication this wrong entry could also block the real
series, because it already occupied the same position slot.

Compare with `<` in the approximate scan and in the MESSI L2 leaf scan, so a
distance that is not strictly better than the current k-th is never inserted.

On 200k clustered 128-d vectors, k=10: returned (id, distance) pairs whose
distance does not match the series: 8 of 50000 before, 0 after; fraction of the
true top-10 returned: 0.9998 -> 1.0000. On 1M vectors the same: 1.0000.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Co-authored-by: sophisid <55842445+sophisid@users.noreply.github.com>
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