Function lower_isotropic_elasticity_cartesian_2d 

pub fn lower_isotropic_elasticity_cartesian_2d(
    program: &KernelProgram,
) -> Result<IsotropicElasticityCartesianModel2d, Diagnostic>

Lower the exact canonical 2D isotropic-elasticity subset.

This lowerer is identity-parametric: it recognizes typed structure rather than source names. It admits exactly one Cartesian box, one spatial-vector displacement, one scalar load-potential definition, one balance Relation, and one exact direct or conserving-interface boundary law on every side. Closed zero trace/traction terminals normalize to method-neutral dispositions; live bindings remain explicit for a later Realization gate.

Errors

Returns EQ0703 when the admitted Model is ambiguous or differs from this deliberately narrow semantic subset.