Avoid storing unnecessary kernel outputs

[AhmedSalih3d]

Summary

• Allocate particle kernel arrays only when explicitly requested
• Skip unpacking and exporting kernel data when unused
• Derive default output variables from KernelOutputMode

Testing

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

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https://chatgpt.com/codex/tasks/task_e_68c57462fc5883239c6c73d49c867ee0