⚡ Bolt: [performance improvement] O(N) K-Nearest Neighbors voting

[teerthsharma]

💡 What: Replaced the $O(K \times N)$ partial bubble sort inside KNNClassifier::predict with slice::select_nth_unstable_by.
🎯 Why: Finding the top $K$ nearest neighbors previously sorted all distances partially, making it slow for large $K$ and $N$.
📊 Impact: Reduces nearest neighbor voting complexity from $O(K \times N)$ to $O(N)$ average time.
🔬 Measurement: Verify by checking the speed of KNNClassifier::predict with a large number of neighbors ($K$) and training points ($N$). Test suite continues to pass (with known exceptions).

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PR created automatically by Jules for task 14029399032835280077 started by @teerthsharma

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