integration: flagship cross-scale gradient — d(as-built bend angle)/d(potential param)

[ecto]

ONE integration test chaining electrons → potentials → MaterialCard → PressBrake: the derivative of an as-built quantity w.r.t. a microscopic parameter, with σ and a receipt chain terminating at the fundamental-constants node. This test IS the landing-page sentence.

Blocked by #494, #497, #502.