`SingleMortar` nodal normal shape derivatives miss reference-normalization dependence

Nobody has claimed this yet.

Currently, `EdgeAvgNodalNormal` computes nodal normals using current coordinates for the numerator but reference coordinates for normalization:

- numerator depends on current coordinates x
- denominator depends on reference coordinates xref

Smith now registers shape-displaced reference coordinates with Tribol, so the evaluated normal uses xref = X + shape. That improves value consistency, but the Enzyme Jacobian path treats xref as constant in `ElementEdgeAvgNodalNormalJacobian()`. As a result, derivatives through the reference-normalization factor are not included.

This means shape derivatives can include the path through current coordinates but miss the path through the reference-coordinate normalization:

x = X + shape + u
xref = X + shape
n = n(x, xref)

The current derivative effectively captures dn/dx, but not dn/dxref.

Simplest fix I can think of is to let nodal normals use current coordinates in the denominator, so d/d(shape) matches d/du, but this will be inconsistent with Puso and Laursen (2004).

Start at ElementEdgeAvgNodalNormalJacobian() and compare it with EdgeAvgNodalNormal(), focusing on the reference-coordinate normalization described in the issue. Determine the intended derivative behavior consistently with Puso and Laursen (2004); the work is done when shape derivatives include the relevant xref dependence and the behavior is validated.

Written by the indexing model from the issue text.