Fix neighbor rebuild storage and add guard/test

[AhmedSalih3d]

Motivation

• Repeated neighbor-list rebuilds could corrupt or index past UniqueCells when every particle occupies its own cell, causing failures during simulations and tests.

Description

• Allocate one extra slot for UniqueCells in src/SPHCellList.jl so there is always a sentinel/dummy cell available for the rebuild path.
• Add a defensive size check in UpdateNeighbors! in src/SPHNeighborList.jl to throw a clear ArgumentError if UniqueCells is undersized instead of silently corrupting state.
• Add a regression test in test/runtests.jl that repeatedly rebuilds the neighbor list with all particles in unique cells to exercise the sentinel-aware path.
• Update existing tests to use the current Δt API (scalar max-acceleration form) and to pass the velocity buffer to FullTimeStep so the test suite matches the current APIs.

Testing

• Ran the full test-suite with julia --project=. -e 'using Pkg; Pkg.test()' and the test run completed successfully with all tests passing.
• Exercised the specific neighbor-rebuild scenario with a focused one-liner test (julia --project=. -e 'using Test, SPHExample, StructArrays, StaticArrays; @testset "neighbor rebuild with all unique cells" begin ... end') and it passed.
• The regression test added to test/runtests.jl was executed as part of Pkg.test() and validated the fix.

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Codex Task