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 <[email protected]>
Co-authored-by: Claude Opus 5.5 <[email protected]>

assets/icons/mesh.planar.svg

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="24" height="24" viewBox="0 0 24 24">
+  <title>mesh.planar</title>
+  <g id="Layer_1" data-name="Layer 1">
+    <path d="M3 5 L21 5 L21 19 L3 19 Z" fill="#2d4e70" stroke="#dde2ea" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+    <path d="M9 5 L9 19 M15 5 L15 19 M3 12 L21 12" fill="none" stroke="#4f9ce8" stroke-width="1.25" stroke-linecap="round" stroke-linejoin="round"/>
+  </g>
+</svg>

assets/icons/src/mesh.planar.json

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+{"shapes": [{"d": "M3 5 L21 5 L21 19 L3 19 Z", "fill": "soft", "stroke": "line"}, {"d": "M9 5 L9 19 M15 5 L15 19 M3 12 L21 12", "stroke": "accent", "width": 1.25}]}