Update adaptive half timestep

[AhmedSalih3d]

Motivation

• Prevent unphysical density oscillations when OutputTimes is large by ensuring the predictor half-step stays consistent with the adaptively-updated full timestep dt.

Description

• Recompute the predictor half-step inside the inner simulation loop by inserting dt₂ = dt * 0.5 at the top of the while SimMetaData.TotalTime <= NextOutputTime block in src/SPHCellList.jl so the half-step tracks the current adaptive dt.

Testing

• Ran julia --project=. -e 'using SPHExample; println("loaded")' which loaded the package successfully; git diff --check returned clean; running the test suite with julia --project=. -e 'using Pkg; Pkg.test()' produced an unrelated test error because test/runtests.jl calls Δt with an outdated 5-argument signature, not caused by this change.

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Codex Task