Replace cell Dict with vector-backed CellLookup for neighbor lists

[AhmedSalih3d]

Motivation

• Replace the per-cell Dict with contiguous vector-backed lookup arrays to reduce allocations and improve performance and future GPU portability.
• Update neighbor-list construction and all cell-index lookups to use a stable, allocation-free interface.

Description

• Added a CellLookup{D} mutable struct and helpers: InitializeCellLookup, ResetCellLookup!, GetCellIndex, SetCellIndex!, and an inline HashCellIndex implementation, plus capacity/resize helpers in src/SPHNeighborList.jl.
• Replaced Dict{CartesianIndex,Int} usage with CellLookup across neighbor-list code and simulation flow, updating function signatures and call sites in src/SPHNeighborList.jl and src/SPHCellList.jl (e.g. UpdateNeighbors!, BuildNeighborCellLists!, neighbor loops, MDBC handling, and RunSimulation setup).
• Adjusted probing wrap arithmetic in the hash loop and added a power-of-two sizing strategy with mask-based indexing to keep lookups branch-friendly and allocation-minimal.
• Commands executed during the change include rg, sed, apply_patch (multiple), git add, and git commit (used to inspect and modify files and record the change).

Testing

• No automated tests were executed for this change.

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