AMR ghost cell support and blanking fixes

Ghost cell support in vtkAMReXGridReader

vtkAMReXGridReader now reads ghost cells from AMReX plotfiles that were written with nGrow > 0. When ghost cells are present in the plotfile, the reader creates grids that are larger than the AMR box by nGrow cells on each side. The grid extent uses the convention [-nGrow, validCells + nGrow] per dimension so that extent index 0 corresponds to the AMR box lo corner. A vtkGhostType array is generated marking ghost cells with the DUPLICATECELL flag.

Previously, ghost cells in AMReX plotfiles were not supported (marked with a TODO comment in the code). Reading a plotfile with ghost cells could result in data misalignment or crashes because the FAB data on disk was larger than the grid allocated by the reader.

AMR cell blanking fix in vtkAMRUtilities

vtkAMRUtilities::BlankGridsAtLevel now correctly handles grids with ghost cells when computing cell blanking (the REFINEDCELL flag). The blanking code converts AMR global coordinates to extent coordinates and uses vtkStructuredData::ComputeCellIdForExtent, which correctly accounts for grids whose extent does not start at zero. Previously, the blanking code used vtkAMRBox::GetCellLinearIndex which assumed the grid dimensions matched the AMR box dimensions, producing incorrect cell IDs when ghost cells were present.

Parent-child relationship fix in vtkOverlappingAMRMetaData

vtkOverlappingAMRMetaData::CalculateParentChildRelationShip now correctly finds parent-child relationships when the number of ghost cells is not a multiple of the refinement ratio. The spatial binner used for accelerating parent-child detection now clamps bin indices to valid ranges using signed arithmetic. Previously, when a child box at the fine level extended beyond the refined parent box bounds (which happens when nGrow < refinementRatio), the unsigned arithmetic in the bin index computation could wrap around, causing the binner to miss valid parent-child relationships entirely. This resulted in no blanking being applied.