Optimize Double.countPairs using frequency map

[alexstaicu35]

Summary

Optimize Double.countPairs by replacing nested loops with a frequency map, reducing complexity from O(n^2) to O(n) while preserving behavior (count distinct values occurring exactly twice).

Changes

• Replace nested loops with HashMap-based frequency counting
• Return direct count of entries with frequency == 2 (remove divide-by-2)
• Add java.util.Map and java.util.HashMap imports

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changeset_id: 1ac36dab-f26a-42ea-b9ee-a509f932bce3

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