Make half-step wrapper views array-like and integrate into time-stepping

[AhmedSalih3d]

Motivation

• Fix MethodError when using Base array utilities (e.g. keys) on half-step wrapper views and make them interoperable with existing array-based code paths.
• Integrate half-step position/velocity/density views into the symplectic time-stepping so neighbor list updates and pressure/density calculations operate on computed half-step values.

Description

• Add HalfStepPosition, HalfStepVelocity, and HalfStepDensity types as subtypes of AbstractVector with convenience constructors, length/axes/size/IndexStyle, and getindex implementations in src/SPHCellList.jl so they behave like array views.
• Use the half-step views throughout the time-stepping loop by creating PositionHalfStep, passing HalfStepPosition/HalfStepVelocity/HalfStepDensity into NeighborLoopPerParticle!, and updating calls that compute pressure and density to use half-step values.
• Add PressureHalfStep! and a new DensityEpsi! overload that accept half-step inputs and motion limiters, and replace usages accordingly.
• Adjust allocation and initialization in SimulationLoop to create AccelerationMax, dρdtI, ∇Cᵢ, and ∇◌rᵢ with similar/fill! and handle NoShifting mode.
• Update SPHNeighborList to resize ParticleRanges in UpdateNeighbors! and add an alternative allocation-free UpdateΔx! overload that computes norms via dot for generic posₙ⁺ types.
• Small example and I/O tweaks in example/StillWedgeMDBC.jl to use mkpath and a local OutputDirectory variable.

Testing

• No automated tests were run for these changes.

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Codex Task