Function build_radius_grid_aoso_gpu 

pub fn build_radius_grid_aoso_gpu(
    runtime: &WgpuRuntime,
    positions: &GpuAoSoXyzBuffer,
    radius: f32,
) -> SpatialResult<GpuAoSoRadiusGrid>

Builds a sparse uniform radius grid directly from retained AoSoA positions.

Cell keys, sorting, and segment compaction stay on the GPU. Grid keys use a zero origin and floor(position / radius); negative coordinates therefore remain valid without requiring CPU-side bounds discovery.