Sweep two directions and the true transport traces a curved surface in output space. Its tangent plane at the fit point is flat — that's the Jacobian lens. The surface bows away from it, and the height of the bow is the curvature: the lens's error. Flat = trustworthy, steep = the lens lies. Drag to orbit. real GLSL
drag = orbit · scroll = zoom
cross-section (2D): the gap along one direction — a parabola, gap = κt²
what you're orbiting: real data — F(x₀ + s·δ₁ + t·δ₂) of a from-scratch transformer, projected into the tangent plane (flat) plus height along the principal curvature normal. the bowl IS the second fundamental form. the reading: at the center (fit point) surface and plane touch — the lens is exact. moving out, the gap opens as ~κt² (see the 2D slice). the radius where the gap stays small is the lens's trust radius. real WebGL: native browser API, inline GLSL, no libraries, fully offline. if the GL context can't be created, this page fails loud with a diagnostic — no dead canvas.