Implements CultGeometrySurfaceNets.Extract (welded CultGeometryQuadMesh, one
quad per interior crossing grid edge, outward-oriented winding) and
CultGeometryQuadNormals.FaceWeighted (unnormalized diagonal-cross-product
area weighting, matching CultGeometryIsoSurface's sign convention and
degenerate-normal rule). Fresh implementation per SURFACE-NETS-PROVENANCE.md;
no code or tables copied from the cited literature.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
