Add SIMD helpers to SPHCellList

[AhmedSalih3d]

Motivation

• Provide a SIMD-friendly implementation for frequent small dot products in the neighbor interaction kernels to attempt a measurable speedup in SPH cell-list compute hotspots.

Description

• Add SIMD as a project dependency in Project.toml and import Vec into src/SPHCellList.jl.
• Introduce DotSimd helpers specialized for SVector{2, T} and SVector{3, T} using SIMD.Vec, with a generic fallback to dot for other sizes.
• Replace hot dot calls in the neighbor interaction kernels (ComputeInteractionsPerParticle!, ComputeInteractionsPerParticleNoKernel!, ComputeInteractionsMDBC!, and related locations) with DotSimd to enable the SIMD path for 2D/3D vectors.

Testing

• Executed julia -e 'using Pkg; Pkg.activate("."); Pkg.add("SIMD")' which installed SIMD and precompiled packages successfully (precompilation completed without error).
• No project unit tests or benchmarks were run as part of this change.

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Codex Task