Parallelize neighbor list construction with OpenMP

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Pull request overview

This PR parallelizes neighbor list construction in BinManager using OpenMP while preserving serial allocation into the existing NeighborList storage.

Changes:

• Adds an OpenMP-parallel neighbor discovery phase.
• Stores discovered neighbor IDs per atom before serially committing them into NeighborList.
• Keeps allocator usage serial because the page allocator is not thread-safe.

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