[SolidMechanics][FEM] Fix FastTetrahedralCorotationalForceField stiffness matrix assembly

[th-skam]

A bit of an obscure bug.

In FastTetrahedralCorotationalForceField, due to a missed transpose, the mechanics change if the edge indexing changes. This is observed in case the solver template triggers the buildStiffnessMatrix path. The other paths are ok.

This can happen in a scene such as the one below. In one case, the topology uses the edges computed by the mesh loader (unstable), in the other case, it re-computes them (stable). The two components register edge indices differently. This also raises another point: do we want to have components that create element arrays in different ways?

    root.addObject("ConstraintAttachButtonSetting")
    root.addObject("VisualStyle", displayFlags="showVisualModels")
    root.addObject("DefaultAnimationLoop")

    for i in range(2):
        with root.addChild("Liver"+str(i)) as liver:
            liver.addObject("EulerImplicitSolver")
            liver.addObject("EigenSimplicialLDLT", name="LinearSolver", template='CompressedRowSparseMatrixMat3x3d')
            liver.addObject("MeshGmshLoader", name="meshLoader", filename="mesh/liver.msh", scale3d=[0.08, 0.08, 0.08], translation=[0, -0.3, -0.2])

            # Explicitly copy position & tetrahedra VS sourcing the loader
            if(i==0):
                liver.addObject("TetrahedronSetTopologyContainer", name="TetraContainer", position="@meshLoader.position", tetrahedra="@meshLoader.tetrahedra")
            else:
                liver.addObject("TetrahedronSetTopologyContainer", name="TetraContainer", src="@meshLoader")

            liver.addObject("MechanicalObject", name="mstate_gel", template="Vec3d")
            liver.addObject("FastTetrahedralCorotationalForceField", name="FF", youngModulus=4e4, poissonRatio=0.3)
            liver.addObject("MeshMatrixMass", name="Mass",totalMass=0.01)
            liver.addObject("BoxROI", name="BoxROI", box=[-0.350, -0.280, -0.360, 0.130, 0.130, -0.200 ])
            liver.addObject("RestShapeSpringsForceField", stiffness=1e3, angularStiffness=1e3, points="@BoxROI.indices")

            with liver.addChild("Visual") as visual:
                color=[]
                if(i==0):
                    color=[0, 1, 0, 1]
                else:
                    color=[1, 0, 0, 1]
                visual.addObject("OglModel", color=color)
                visual.addObject("IdentityMapping")

Screen.Recording.2026-06-15.133446.mp4

[ci-build][with-all-tests]

[th-skam]

[ci-build][with-all-tests]