2D beam MMS: plane stress validation under distributed load (SOFA vs FreeFEM)

[Fimache]

Summary

Adds the plane stress counterpart to the existing plane strain distributed-load
validation for the 2D cantilever beam. No analytical solution is available for
this loading case, so validation is performed via cross-comparison between SOFA
and FreeFEM++ on the shared beam2d_tri.msh mesh.

• Elements: P1 triangles (beam2d_tri.msh)
• Load case: constant self-weight-type distributed load (0, -q) over the domain
• Plane stress: obtained on the SOFA side via LinearSmallStrainFEMForceField
  with template="Vec2d" , and on the FreeFEM side via lambda = E*nu / (1 - nu^2) in the weak form.
• BCs: fixed (Dirichlet) on the left edge (label 1); distributed load applied
  as consistent nodal forces (1/3 per node per triangle, integrated over element area).

Results

| Quantity | RMS (SOFA vs FreeFEM) |
| u_x | 1.006825e-12 |
| u_y | 5.512254e-12 |

[th-skam]

Ok works