Degree 0 support for curved meshes

Currently, the degree 0 case, which reduces to the finite volume method, only works for TreeMesh. For StructuredMesh, it is currently giving me NaN values. I think that having degree 0 support will be really good, as it is robust.

Start by reproducing the NaN values for degree 0 on StructuredMesh and compare that behavior with the existing TreeMesh finite-volume path. Trace the degree-0 curved-mesh setup and identify where StructuredMesh diverges; done means degree 0 runs on StructuredMesh without NaN values and retains the intended finite-volume behavior.

Written by the indexing model from the issue text.