[FIX] Sparse initial pivot

[nathanneike]

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• Performance improvement

Motivation and context / Related issue

Sparse EMD solver was 10-40x slower than dense solver on full matrices due to O(n³) complexity bug in arc iteration. The nextOut() and nextIn() methods in sparse_bipartitegraph.h were performing linear search through arc lists on every call during pivot selection.

How has this been tested (if it applies)

• Rebuilt C++ extension with changes
• All existing EMD/LP tests pass (1031 tests)

PR checklist

• I have read the CONTRIBUTING document.
• The documentation is up-to-date with the changes I made (check build artifacts).
• All tests passed, and additional code has been covered with new tests.
• I have added the PR and Issue fix to the RELEASES.md file.

[codecov]

Codecov Report

❌ Patch coverage is 88.23529% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.07%. Comparing base (9043960) to head (184acd3).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #785      +/-   ##
==========================================
- Coverage   97.07%   97.07%   -0.01%     
==========================================
  Files         107      107              
  Lines       22142    22156      +14     
==========================================
+ Hits        21495    21507      +12     
- Misses        647      649       +2     

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