Boundary distance determines geodesic scattering

With the full boundary metric fixed, first variation gives $d_x{d}_g(w)=-⟨v_{\mathrm{in}},w⟩$ and $d_y{d}_g(z)=⟨v_{\mathrm{out}},z⟩$ for tangential endpoint variations. These covectors recover the tangential parts of both unit velocities. Their normal components follow from unit length and the inward/outward sign, so equal boundary distances give equal scattering relations. Without a fixed boundary metric, the bundle identification must first be gauged by a boundary-fixing diffeomorphism.