Half-step wrappers, neighbor-list robustness, UpdateΔx fallback, and make StillWedge example portable

[AhmedSalih3d]

Motivation

• Reduce per-step allocations and enable neighbor/kernel code to operate on on-demand half-step views to support symplectic-like stepping semantics.
• Preserve existing clamping and density-update semantics while allowing time-stepping to use half-step values without method errors.
• Prevent neighbor-list failures from too-small ParticleRanges and accept lazy/wrapper-backed position-like inputs for UpdateΔx!.
• Make the StillWedgeMDBC example runnable outside a Windows environment and remove an undefined profiling macro.

Description

• Added HalfStepPosition, HalfStepVelocity, and HalfStepDensity wrapper types with IndexStyle/axes/size/getindex implementations in src/SPHCellList.jl to compute half-step values on demand and passed those wrappers into neighbor calls.
• Moved support buffers (e.g. dρdtI, AccelerationMax, gradient buffers) into the main SimulationLoop and removed per-step temporaries, wired PressureHalfStep! and a new DensityEpsi! overload into the time-stepping sequence in src/SPHCellList.jl and src/SimulationEquations.jl.
• Made UpdateNeighbors! defensive by resizing ParticleRanges to length(Particles) + 2, setting the sentinel entry to length(Particles) + 1, and forcing a rebuild when SimMetaData.IndexCounter == 0 in src/SPHNeighborList.jl.
• Imported LinearAlgebra.dot and added a generic UpdateΔx! fallback that accepts wrapper-backed position-like inputs and computes squared norms using dot in src/SPHNeighborList.jl.
• Exported PressureHalfStep! and added the accompanying half-step pressure/density helper functions in src/SimulationEquations.jl so callers can invoke them.
• Made example changes in example/StillWedgeMDBC.jl to remove @profview, use a local OutputDirectory = joinpath(pwd(), "output", "StillWedge2D_MDBC"), and create it with mkpath so the script runs on non-Windows systems.

Testing

• Ran julia --project=. -e 'include("example/StillWedgeMDBC.jl")', which initially failed due to @profview being undefined, and then succeeded to start after removing the macro and making the output directory portable.
• Precompilation completed successfully and the example produced simulation startup output (particle counts, kernel and metadata), but the full simulation is long-running and was interrupted during manual testing.
• No automated unit tests were executed for these changes.

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Codex Task