Add WCSPH shifting corrections (density/velocity gradient accumulation and application)

[AhmedSalih3d]

Motivation

• Shifting currently only accumulates particle-position correction vectors without applying the WCSPH-style density/velocity corrections, which can leave post-shift density/velocity inconsistent.
• The free-surface shifting logic needs to remain dimension-aware and provide a guardable free-surface mode so corrections only apply when intended.
• Support buffers and neighbor-loop accumulation must be extended so the full-time update can apply consistent density and velocity corrections after shifting.

Description

• Extend support allocations in src/PreProcess.jl to return new gradient buffers ∇ρᵢ and ∇uᵢ for simulations with shifting.
• Update neighbor-loop APIs and call sites in src/SPHCellList.jl to accept the additional gradient buffers and to accumulate per-pair contributions to density_grad_acc and velocity_grad_acc in both kernel and no-kernel interaction paths.
• Modify FullTimeStep in src/TimeStepping.jl to accept the new buffers and apply WCSPH-style corrections by updating Density[i] += dot(δxᵢ, ∇ρᵢ[i]) and Velocity[i] += (∇uᵢ[i] * δxᵢ) after computing the shift δxᵢ, while preserving the free-surface guarded shifting behaviour and adding dimension-aware FreeSurfaceThreshold helpers.
• Add FreeSurfaceMode/InternalFlow/FreeSurfaceCorrection types in src/SimulationMetaDataConfiguration.jl and re-export the symbols from src/SPHExample.jl, and update the isolated-particle test in test/runtests.jl to unpack the expanded allocation tuple.

Testing

• Unit tests were updated (the isolated-particle test now accepts ∇ρ/∇u from AllocateSupportDataStructures), but no automated test suite was executed as part of this change.
• Commands used for local inspection during the change included repository searches and file previews such as rg -n "∇ρᵢ" src and nl -ba src/TimeStepping.jl | sed -n '120,220p' to verify call-site and function signature changes.

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