Function insert_particles 

pub fn insert_particles(
    cl: &mut GpuCellList,
    new_positions: &[[f64; 3]],
) -> usize

Insert particles into an existing cell list without a full rebuild.

This is a sequential incremental insert that updates cell_counts and sorted_indices in place. The cell_starts offsets remain valid only if the new particles land in cells that already have capacity; otherwise a full rebuild should be used.

Returns the number of particles successfully inserted.