Function verts_per_chunk 

pub fn verts_per_chunk(res: u32) -> u32

Total vertex count for one chunk at resolution res = interior grid + perimeter skirt (Phase 3 crack fix). This is the per-slot stride of the vertex pool, the verts_tex/pos_tex sizing unit, and the realize dispatch count. The interior block (L < interior_verts_per_chunk(res)) is unchanged; skirt verts are appended at L >= interior_verts_per_chunk(res).