Add SIMD-accelerated density update for DensityEpsi!

[AhmedSalih3d]

Motivation

• Speed up the per-particle density adjustment in the symplectic time stepping path by vectorizing the loop with SIMD operations.
• Add SIMD.jl as a direct dependency to enable portable explicit SIMD intrinsics.

Description

• Replaced the scalar loop in DensityEpsi! with a call to a new _density_epsi_simd! helper that uses SIMD.Vec, vload and vstore! and falls back to a scalar tail loop.
• Added using SIMD to src/SimulationEquations.jl and implemented lane selection based on eltype(Density) (8 lanes for Float32, 4 otherwise).
• Updated Project.toml to add SIMD to [deps] and [compat] and produced a resolved Manifest.toml during Pkg.resolve/Pkg.instantiate.
• DensityEpsi! now returns Density after the SIMD update helper completes.

Testing

• Ran julia --project=. -e 'using Pkg; Pkg.resolve()', julia --project=. -e 'using Pkg; Pkg.instantiate()' and julia --project=. -e 'using Pkg; Pkg.test()' to validate dependency resolution, instantiation and tests.
• Pkg.resolve() emitted a registry download 403 warning in this environment but SIMD was installed and the manifest was updated successfully.
• Test results: the package precompiled and SPHExample tests passed (time stepping and isolated particle tests completed successfully).
• No automated test failures were observed after the change.

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Codex Task