Preserve adaptive timestep across output blocks

[AhmedSalih3d]

Motivation

• Output scheduling was resetting the integrator timestep at each OutputTimes boundary, making the I/O cadence influence the numerical solution instead of only the logging/export behavior.

Description

• Continue from the last adaptive timestep by initializing dt from SimMetaData.CurrentTimeStep when entering SimulationLoop instead of always using the CFL baseline.
• Recompute the half-step dt₂ inside the inner time-stepping loop so it follows any adaptive dt changes within the block.
• Store the updated adaptive timestep back into SimMetaData.CurrentTimeStep after each solver iteration so the next output block resumes with the same timestep.

Testing

• Ran julia --project=. -e 'using SPHExample' which precompiled and loaded the package successfully.
• Ran git diff --check and there were no whitespace/format issues.
• Ran julia --project=. -e 'using Pkg; Pkg.test()' which exercised the test suite and produced an error in the existing time-stepping test: the test calls Δt(pos, vel, acc, sc, ker) but only Δt(max_acceleration, SimulationConstants, SPHKernel) is defined, so the failure is in the test API usage rather than this change.

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Codex Task