Fix Windows OpenMP wheel build

[eggpig]

Summary

• replace MSVC-incompatible #pragma omp atomic update with the portable OpenMP atomic form
• use signed loop indices for OpenMP parallel for loops in modularity helpers
• remove the unsupported merge-multiple input from the cibuildwheel step

Verification

• python3 -m pip wheel . --no-deps -w /tmp/easygraph-wheel-openmp-fix
• git diff --check

Notes

The original failing workflow was Build,download and publish wheels run 27082170598. The hard failure was in the Windows cp38-win32 wheel build with MSVC OpenMP errors C7660 and C3016. I did not manually rerun the publish workflow because it includes a PyPI publish job after wheel builds complete.

[eggpig]

CI follow-up from this fix:

• The previous Windows MSVC OpenMP hard error is fixed in PR code review: both build (windows-latest, 3.9) and build (windows-latest, 3.12) passed the Build and install step.
• Those jobs still fail later at the existing Test step because .github/workflows/pr_code_check.yaml runs python tests/test.py, but tests/test.py does not exist in this repository.
• pre-commit is also still failing on existing all-files formatting changes outside this PR's C++/wheel scope.
• The pytest workflow still fails on the current baseline tests noted separately.

So this PR addresses the Windows OpenMP wheel/build hard failure specifically; the remaining red checks are pre-existing CI baseline/workflow issues.