Add non-sorting neighbor list option with per-cell particle order buffer

[AhmedSalih3d]

Motivation

• Provide an alternative neighbor-list rebuild that avoids re-sorting the particle arrays so particle storage remains stable (useful for external references or when reordering is expensive).
• Allow building a per-cell particle ordering buffer so neighbor iteration can proceed without mutating particle storage.
• Make the non-sorting path selectable at runtime and thread the ordering through neighbor loops and MDBC to preserve behavior.

Description

• Added UpdateNeighborsNoSort! to src/SPHNeighborList.jl which builds per-cell ParticleOrder and CellOffsets buffers instead of sorting particles, and exported the new symbol.
• Threaded optional ParticleOrder into neighbor loops, MDBC handling, and the main simulation loop in src/SPHCellList.jl, including a small ResolveParticleIndex helper to map indices when an order buffer is used.
• Added ParticleOrder and CellOffsets allocations and a UseSortingNeighborList::Bool switch to SimulationMetaData in src/SimulationMetaDataConfiguration.jl, exposed the new option in SPHExample exports, and documented the switch in README.md.
• Updated exports and call sites so either the original sorting path (UpdateNeighbors!) or the new no-sort path (UpdateNeighborsNoSort!) is used depending on SimMetaData.UseSortingNeighborList.

Testing

• No automated tests or CI jobs were executed for this change (none were run).

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Codex Task