Make time-stepping mode a SimulationMetaData type parameter

[AhmedSalih3d]

Motivation

• Avoid runtime abstract-field inference loss from storing TimeSteppingMode as an abstract field so compiled inner loops can reason about the time-stepping strategy at compile time.
• Allow the time-stepping mode to be selected conveniently via existing short SimulationMetaData constructors while ensuring the concrete mode is available to performance-sensitive code paths.

Description

• Add a TMode<:TimeSteppingMode type parameter to SimulationMetaData and replace the runtime TimeSteppingMode field with a concrete TimeSteppingMode::Type{TMode} stored value in src/SimulationMetaDataConfiguration.jl.
• Provide helpers (_time_stepping_type, _metadata_kwargs_without_time_stepping_mode) and a SimulationMetaData{...}(; kwargs...) constructor that accepts TimeSteppingMode as either a type or instance and yields a SimulationMetaData specialized on the chosen TMode.
• Update the simulation entry points in src/SPHCellList.jl to derive TimeSteppingType = typeof(SimTimeStepping) in RunSimulation, pass that Type into SimulationLoop, and branch on the compile-time TMode (if TMode <: SingleNeighborTimeStepping / if TMode <: SymplecticTimeStepping) instead of reading an abstract field.
• Preserve existing short constructors for other metadata combinations and keep the default mode as SingleNeighborTimeStepping when not specified.

Testing

• Ran julia --project=. -e 'using SPHExample; meta=SimulationMetaData{2,Float64}(SimulationName="x", SaveLocation=".", TimeSteppingMode=SymplecticTimeStepping()); @show typeof(meta); @show meta.TimeSteppingMode; meta2=SimulationMetaData{2,Float64}(SimulationName="x", SaveLocation=".", TimeSteppingMode=SingleNeighborTimeStepping); @show typeof(meta2);' which succeeded and showed the metadata specialized on the selected time-stepping types.
• Ran the package test suite with julia --project=. -e 'using Pkg; Pkg.test()' which precompiled successfully but the time stepping test errored due to an existing Δt method-signature mismatch (test calls Δt with five arguments while only a three-argument method exists), so the test run did not fully pass.

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Codex Task