Dispatch time-stepping neighbor loops via multiple dispatch

[AhmedSalih3d]

Motivation

• Replace runtime isa checks for SingleNeighborTimeStepping and SymplecticTimeStepping with multiple-dispatch to let Julia compile specialized paths and keep the main loop concise.
• Move mode-specific neighbor-loop sequences out of SimulationLoop to reduce branching and encourage better performance and code clarity.

Description

• Add InitializeTimeStepNeighborLoop! overloads: a full initializer for SingleNeighborTimeStepping and an inline no-op for SymplecticTimeStepping to handle initial pressure/MDBC/neighbor-loop setup via dispatch.
• Add AdvanceHalfTimeStepNeighborLoop! overloads for SymplecticTimeStepping and SingleNeighborTimeStepping encapsulating the half-step neighbor-loop sequences and motion/pressure updates.
• Replace the isa branches in SimulationLoop with calls to InitializeTimeStepNeighborLoop! and AdvanceHalfTimeStepNeighborLoop!, keeping shared loop structure and letting Julia choose the mode-specific implementation.
• All changes are contained in src/SPHCellList.jl.

Testing

• Ran git diff --check which reported no issues.
• Ran julia --project=. -e 'using SPHExample', which precompiled dependencies and loaded the package successfully.
• Both automated checks completed successfully.

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Codex Task