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/features.md

@@ -5,7 +5,7 @@ Generated from the capability catalogue (`crates/engine/src/catalogue.rs`) by
 Every entry is a toolbar button. **Ready** entries run their command; **coming soon** entries are
 shown greyed with the roadmap milestone that brings them (ROADMAP.md).
 
-45 ready, 55 coming soon.
+46 ready, 55 coming soon.
 
 ## Geometry
 
@@ -30,6 +30,7 @@ shown greyed with the roadmap milestone that brings them (ROADMAP.md).
 | Tet | Tet Mesh — Constrained Delaunay tet mesh of a body: every boundary triangle kept, faces tagged (refinement comes next) | ready | `mesh.tet.generate` |
 | Tet | Quadratic — Second-order elements with mid-side nodes on the geometry | coming soon (M2) |  |
 | Tet | Delaunay Points — Delaunay tetrahedralization of a point list (debugging aid) | ready | `mesh.delaunay.points` |
+| 2D | 2D Mesh — Quad or triangle mesh of a planar face or polygon in the XY plane for plane stress, plane strain and axisymmetric models; edges tagged | ready | `mesh.planar` |
 | Structured Hex | Quad Faces — All-quad meshes of faces on shared edge nodes | coming soon (M5) |  |
 | Structured Hex | Mapped Hex — Mapped hex mesh of six-sided regions | coming soon (M5) |  |
 | Structured Hex | Sweep Hex — Hex mesh swept along extrusions and revolutions | coming soon (M5) |  |
@@ -60,7 +61,7 @@ shown greyed with the roadmap milestone that brings them (ROADMAP.md).
 | Materials | Damage — Damage initiation and evolution | coming soon (M8) |  |
 | Materials | Thermal — Conductivity, specific heat and expansion | coming soon (M8) |  |
 | Materials | Equation of State — Polynomial, ideal gas, Mie–Grüneisen, JWL and tabulated equations of state | coming soon (M11) |  |
-| Sections | Section — Solid, shell, beam or rod section assigned to a set, with its element formulation | ready | `model.section.create`, `model.section.edit`, `model.section.delete` |
+| Sections | Section — Solid, shell, beam, rod or 2D (plane stress, plane strain, axisymmetric) section assigned to a set, with its element formulation | ready | `model.section.create`, `model.section.edit`, `model.section.delete` |
 | Sections | Composite — Composite layups and continuum shells | coming soon (M8) |  |
 | Sections | Orientation — Material orientations | coming soon (M8) |  |
 | Sets | Set — A set of nodes, elements and faces, or of named geometry that survives remeshing | ready | `model.set.create`, `model.set.edit`, `model.set.delete`, `model.set.resolve` |