Per-vertex AO on a half-block grid

Every regenerate builds a new random terrain and re-meshes both chunks. Stairs, slabs, fences and walls crease from half-cell probing; glass panes and the door never self-shade; the two chunks are meshed independently with a 1-cell halo, so AO stays seamless across the border (dashed white line). The AO slider is a shader uniform: no re-mesh.