Validate particle positions/accelerations and preserve adaptive timestep across outputs

[AhmedSalih3d]

Motivation

• Ensure simulation stability by validating particle positions and acceleration magnitudes before using them in neighbor extraction and timestep calculation.
• Keep the adaptive integrator continuous across output chunks so the timestep does not reset at output boundaries.
• Avoid unnecessary neighbor-list rebuild work during initialization and re-run necessary single-neighbor initializations only when neighbor cells are (re)computed.

Description

• Added input validation in ExtractCells! to throw a DomainError when a particle position contains non-finite values and removed an unsafe @simd ivdep use to make the loop clearer (src/SPHNeighborList.jl).
• Introduced an overloaded Δt that computes max acceleration from vectors of acceleration and added an initial_time_step helper that preserves SimMetaData.CurrentTimeStep across output chunks; exported initial_time_step (src/TimeStepping.jl).
• Added a safety check in UpdateTimeStep to detect non-finite acceleration magnitudes and throw a DomainError to fail fast on unstable states (src/TimeStepping.jl).
• Added FullTimeStep overloads to route calls consistently for different SimulationMetaData type parameters (src/TimeStepping.jl).
• In the main stepping loop (src/SPHCellList.jl) replaced the unconditional initial timestep calculation with initial_time_step(...), deferred the first neighbor-list build to only when SimMetaData.IndexCounter == 0, and moved the SingleNeighborTimeStepping initialization into that branch.
• When neighbor cells are rebuilt during the run, re-run Pressure!, ApplyMDBCBeforeHalf!, and NeighborLoopPerParticle! for SingleNeighborTimeStepping mode to reinitialize state after the rebuild (src/SPHCellList.jl).
• Recomputed dt₂ after timestep updates to keep the half-step consistent with the new dt value (src/SPHCellList.jl).

Testing

• Ran the package test suite with ] test . which exercised neighbor-list, time-stepping, and integration tests; all tests passed.
• Executed unit tests covering neighbor extraction and timestep update routines to verify DomainError is raised for non-finite inputs and to confirm preserved initial timestep behavior; these tests succeeded.

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Codex Task