Function interior_verts_per_chunk 

pub fn interior_verts_per_chunk(res: u32) -> u32

Interior vertex count for one chunk at resolution res.

The triangular grid with res edge segments has (res+1)*(res+2)/2 vertices (sum of rows 1..=res+1). These occupy local indices L < interior_verts_per_chunk; the realize L→(i,j) decode and the GPU-readback reference both depend on this interior block being byte-identical, so it is split out from the skirt verts.