V.D35 — Readout curvature

The readout curvature kappa_R(n) := d^2 R_d / dn^2 is the second derivative of the distance readout functor with respect to orbit depth. When kappa_R(n) != 0, equal orbit-depth intervals map to unequal SI-length intervals. Scope: conjectural (explicit form deferred to Part V).