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/lib.rs

@@ -53,10 +53,10 @@ pub use check::{Diagnostic, Resolved, ResolvedSet, Severity};
 pub use constraints::{Coupling, CouplingKind, ReferencePoint};
 pub use fluids::{BoxFace, BoxFaces, FaceCondition, FluidBoundary, FluidDomain, Profile, SurfaceBoundary, SurfaceCondition};
 pub use geometry::{Classification, Entity, GeometryRef};
-pub use loads::{Distribution, Load};
+pub use loads::{Distribution, Load, centrifugal_acceleration};
 pub use materials::{Elastic, Fluid, Material};
-pub use mesh::{Element, ElementFace, ElementKind, Node, check_coordinates};
-pub use sections::{BeamProfile, Formulation, Integration, Section, SectionKind};
+pub use mesh::{Element, ElementEdge, ElementFace, ElementKind, Node, check_coordinates};
+pub use sections::{BeamProfile, Extent, Formulation, Integration, Section, SectionKind};
 pub use sets::Set;
 pub use steps::{BulkViscosity, ExplicitDynamic, Hourglass, HourglassKind, InitialVelocity, OutputVariable, Procedure, RigidWall, Step};
 pub use units::UnitSystem;