Skip non-fluid work in neighbor loops

[AhmedSalih3d]

Motivation

• Reduce unnecessary computation for boundary/fixed/moving particles by avoiding full neighbor interaction work when a particle is not Fluid.
• Prevent accumulation of stale values in per-particle buffers for non-fluid entries by explicitly zeroing outputs.

Description

• Added early-exit checks in all NeighborLoopPerParticle! variants to skip the heavy neighbor scanning when ParticleType[i] != Fluid and to write zeroed accumulators back to particle fields.
• Avoided processing non-fluid neighbors by guarding interaction calls with ParticleType[jIndex] == Fluid inside the inner loops.
• Pulled ParticleType = SimParticles.Type once per thread loop to reduce repeated field access.
• Change localized to src/SPHCellList.jl across the multiple overloads of NeighborLoopPerParticle! handling different metadata modes.

Testing

• No automated tests were run on this change (CI/tests not executed).

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Codex Task