Audit UVs

Sweep for lightmap faults that stay invisible until lighting is built. The flagship is a lightmap coordinate index pointing past the end of a mesh's UV channels, which bakes to black or garbage: the field accepts any integer and nothing revisits it when a mesh is re-exported with fewer UV sets. Also lightmaps sharing UV channel 0 with the texture UVs, which bleeds light between unrelated faces, and resolutions that are far off a sensible density for the size of the mesh in either direction.