2D elements: plane stress, plane strain and axisymmetric sections on the DOF framework; mesh.planar (M4) (#39)

Model: planar section kinds with Extent, element edges in sets, edge-weighted 2D loads,
centrifugal load, 2D model checks. fem: Plane kernels on Basis::{Tri3,Tri6,Quad4,Quad8}
declaring ElementDofs::new(DofSet::PLANE); QM6 incompatible modes with the
Wilson-Ibrahimbegovic correction. Solver: Kernel enum on the DofLayout; 3D results
bit-identical (hash-checked). Decks: Abaqus CPS/CPE/CAX, Nastran membranes and RFORCE,
LS-DYNA ELFORM 12/13. Mesh: free triangle mesher, planar region meshing, face boundaries.
Engine: mesh.planar with tagged edges. Analytic verification suite in solver/tests/plane.rs.


Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

Co-authored-by: jyviko <2501648+jyviko@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

docs/solvers/literature.md

@@ -979,6 +979,14 @@ the module docs):
   Lewis, H. D. Simon, SIAM J. Matrix Anal. Appl. 15(1):228–272, 1994; B. N. Parlett, *The
   Symmetric Eigenvalue Problem*, SIAM 1998; G. H. Golub, C. F. Van Loan, *Matrix Computations*,
   4th ed., 2013.
+- 2D elements (plane stress, plane strain, axisymmetric): Zienkiewicz, Taylor & Zhu 2013, ch. 6
+  and §6.6; Hughes 2000, §2.7 and §3.5. Quad4 incompatible modes (QM6): Wilson et al. 1973 and
+  Taylor, Beresford & Wilson 1976 as for hex8; in axisymmetric elements the modes' strain
+  operator is shifted by its volume mean, the patch-test correction of E. L. Wilson and A.
+  Ibrahimbegovic, *Use of incompatible displacement modes for the calculation of element
+  stiffnesses or stresses*, Finite Elements in Analysis and Design 7(3):229–241, 1990. Rotating
+  discs and cylinders for verification: S. Timoshenko, J. N. Goodier, *Theory of Elasticity*,
+  3rd ed., McGraw-Hill 1970, §32.
 - Wedge6: the linear triangle times the linear interval; the symmetric three-point triangle rule
   (Zienkiewicz, Taylor & Zhu 2013, table 6.3) times two-point Gauss–Legendre.
 - Couplings and constraint elimination: rigid links and master–slave elimination by