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/engine/src/cmd/define.rs

@@ -66,7 +66,7 @@ definition_commands!(
     Set,
     "set",
     "Set",
-    r#"{"name": "root", "geometry": [{"body": "Body1", "face": "F2:top"}, {"body": "Body1", "edge": "…"}, {"body": "Body1", "vertex": "…"}, {"body": "Body1"}], "nodes": [labels], "elements": [labels], "faces": [{"element": 1, "face": 3}]} — any of the member lists; geometry members resolve against the mesh's geometry tags after every remesh (a face name also selects its split pieces `name#0`, `name#1`, …)"#,
+    r#"{"name": "root", "geometry": [{"body": "Body1", "face": "F2:top"}, {"body": "Body1", "edge": "…"}, {"body": "Body1", "vertex": "…"}, {"body": "Body1"}], "nodes": [labels], "elements": [labels], "faces": [{"element": 1, "face": 3}], "edges": [{"element": 1, "edge": 2}]} — any of the member lists (element edges: where a 2D model's surface loads act); geometry members resolve against the mesh's geometry tags after every remesh (a face name also selects its split pieces `name#0`, `name#1`, …)"#,
     add_set,
     replace_set,
     remove_set,
@@ -94,7 +94,7 @@ definition_commands!(
     Section,
     "section",
     "Section",
-    r#"{"name": "web", "set": "all", "material": "Steel", "kind": {"type": "solid"} | {"type": "shell", "thickness": 2, "offset": 0 (-0.5..=0.5 of the thickness), "points": 5} | {"type": "beam", "profile": {"type": "rectangle", "width", "height"} | {"type": "circle", "radius"} | {"type": "pipe", "radius", "thickness"} | {"type": "i", "height", "width", "web", "flange"} | {"type": "box", "width", "height", "thickness"}, "orientation": [x, y, z]} | {"type": "rod", "area": 10}, "formulation": {"integration": "full|reduced", "incompatible_modes": false, "hybrid": false}}"#,
+    r#"{"name": "web", "set": "all", "material": "Steel", "kind": {"type": "solid"} | {"type": "shell", "thickness": 2, "offset": 0 (-0.5..=0.5 of the thickness), "points": 5} | {"type": "beam", "profile": {"type": "rectangle", "width", "height"} | {"type": "circle", "radius"} | {"type": "pipe", "radius", "thickness"} | {"type": "i", "height", "width", "web", "flange"} | {"type": "box", "width", "height", "thickness"}, "orientation": [x, y, z]} | {"type": "rod", "area": 10} | {"type": "plane_stress", "thickness": 1} | {"type": "plane_strain", "thickness": 1 (default)} | {"type": "axisymmetric"} (2D: elements in the XY plane; axisymmetric about Y with x the radius, forces totals over the circumference), "formulation": {"integration": "full|reduced", "incompatible_modes": false, "hybrid": false}}"#,
     add_section,
     replace_section,
     remove_section,
@@ -150,7 +150,7 @@ definition_commands!(
     Load,
     "load",
     "Load",
-    r#"{"name": "push", "type": "force|moment", "target": {"set": "…"} | {"reference_point": "…"}, "vector": [x, y, z], "distribution": "total" (default: the vector is the total, split by area over the set's faces, or equally over its nodes) | "per_node" (at every node: depends on the mesh)} | {"name", "type": "traction", "set", "vector" (per area, on the set's faces)} | {"name", "type": "pressure", "set", "magnitude" (positive pushes in)} | {"name", "type": "gravity", "acceleration": [x, y, z], "set" (optional, default the whole model)} | {"name", "type": "body_force", "set", "vector" (per volume)} — active in the steps that list it (model.step.activate)"#,
+    r#"{"name": "push", "type": "force|moment", "target": {"set": "…"} | {"reference_point": "…"}, "vector": [x, y, z], "distribution": "total" (default: the vector is the total, split by area over the set's faces, or equally over its nodes) | "per_node" (at every node: depends on the mesh)} | {"name", "type": "traction", "set", "vector" (per area, on the set's faces)} | {"name", "type": "pressure", "set", "magnitude" (positive pushes in)} | {"name", "type": "gravity", "acceleration": [x, y, z], "set" (optional, default the whole model)} | {"name", "type": "body_force", "set", "vector" (per volume)} | {"name", "type": "centrifugal", "angular_velocity" (rad per time unit), "point": [x, y, z] (on the axis), "axis": [x, y, z], "set" (optional)} — 2D models: in-plane vectors, surface loads on element edges; active in the steps that list it (model.step.activate)"#,
     add_load,
     replace_load,
     remove_load,