Function build_sprite_model 

pub fn build_sprite_model(kv6: &Kv6) -> SpriteModel

Build the DDA volume from a KV6. Columns are packed in x + y*mx order; each column’s voxels are sorted ascending by z so the shader’s popcount-rank colour lookup is correct.

Panics

If the KV6’s ylen counters disagree with voxels.len() (a malformed model).