Fix SPEA2 truncation selection#788
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Fixes #773
This PR updates SPEA2 survival truncation to use lexicographical ordering of sorted distances when too many non-dominated solutions are present.
Previously, the truncation step only considered the nearest-neighbor distance, which could remove an extreme non-dominated solution in tie cases. The updated implementation compares the sorted distance rows lexicographically, as required by the SPEA2 truncation procedure.
A regression test was added for the minimal example from the issue, checking that the extreme points are preserved when reducing three non-dominated points to two survivors.
Test:
python -m pytest tests/algorithms/test_spea2.py -q