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/explicit/src/system.rs

@@ -368,6 +368,17 @@ impl System {
                         }
                     });
                 }
+                Load::Centrifugal { angular_velocity, point, axis, set } => {
+                    let chosen: Option<&[u32]> = set.as_ref().map(|s| resolved.sets.get(s).map(|r| r.elements.as_slice()).unwrap_or_default());
+                    let xyz = &self.xyz;
+                    self.each_element(chosen, |nodes, weights, rho| {
+                        for (node, w) in nodes.iter().zip(weights) {
+                            let x = xyz.get(*node).copied().unwrap_or_default();
+                            let a = modelcraft_model::centrifugal_acceleration(*angular_velocity, *point, *axis, x);
+                            add_at(&mut f, *node, &a.map(|c| rho * w * c));
+                        }
+                    });
+                }
             }
         }
         self.external = f;