Use array-based cell lookup for neighbor search

[AhmedSalih3d]

Summary

• replace hash-based cell dictionary with direct array lookup
• compute linear cell indices once per update and reuse in neighbor loops
• apply array lookup to boundary correction path

Testing

• julia --project=. -e 'using Pkg; Pkg.test()'

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https://chatgpt.com/codex/tasks/task_e_68c574aca9188323988c535e4008fec7