The frozen partial-diagonal class

This is a conditional obstruction, not an evaluated obstruction for the actual absent inherited lift. It applies on the graph domain with the stated contact support. After propagation or pushforward, the image of a local contact can have a larger support; that image must be tracked rather than replaced by the original local contact.

Agreement only on $U_{\mathrm{can}}$ does not prove that two extensions differ by permitted collision contacts. A characteristic-supported difference outside the collision locus can be invisible on that smaller reference domain.