corners and verts are carved out of decoded_ at fixed offsets 6*65536 bytes apart,
and NormalizeVertices fills corners with indexUpperBound - indexLowerBound + 1
SimpleVertex. The vertexCount > 1024 guard doesn't bound that: the index values come
from the game, so 1024 indices can span the full 16-bit range and run corners into
verts, making the cull decision from overwritten data. Bail on an index over 1024
and report visible - a bbox test that large isn't worth doing anyway.