Use dispatch for particle saving

[AhmedSalih3d]

Summary

• use SaveMode and multiple dispatch to handle 2D/3D particle export
• refactor SetupVTKOutput to delegate to dispatch-based save routines

Testing

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

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https://chatgpt.com/codex/tasks/task_e_68c69f94ba088323ab435a5ccad104f3