Initial particle sort by cell ID to improve locality

[AhmedSalih3d]

Motivation

• Improve memory locality and (potentially) runtime performance by reordering particle storage according to initial cell assignment before the main simulation loop.

Description

• Add SortParticlesByCellID!(SimParticles::StructArray, ParticleOrder) which permutes every particle field using foreachfield and then resets ParticleOrder to the identity mapping.
• Invoke the one-time sort immediately after the first UpdateNeighbors! call when SimMetaData.Iteration == 0 and record timing under the label "01b Initial Sort By CellID".
• Leave all other neighbor-list and per-iteration logic unchanged so subsequent neighbor rebuilds continue to use the existing non-sorting update path.

Testing

• No automated tests were run for this change.
• Commands inspected or used while developing the change include rg -n ..., sed -n ..., and nl -ba ..., and the patch was applied to src/SPHCellList.jl.

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Codex Task