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>

crates/model/src/mesh.rs

@@ -195,6 +195,16 @@ pub struct ElementFace {
     pub face: u8,
 }
 
+/// One edge of an element: the element's label and its edge number (1-based,
+/// [`ElementKind::edges`]). In a 2D model (plane stress, plane strain, axisymmetric) the edges on
+/// the boundary are what pressures and tractions act on, as faces are for solids.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Serialize, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct ElementEdge {
+    pub element: u32,
+    pub edge: u8,
+}
+
 /// An element: a topology and its node labels.
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 pub struct Element {