Shells and beams: MITC4/MITC3 shells, Timoshenko beams, six-DOF nodes (M4) (#47)

* fem: MITC4/MITC3 flat shells and Timoshenko beams with profile properties (ElementDofs STRUCTURAL)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* solver: shells and beams on the DofLayout beside solids, 2D elements and trusses; analytic verification and tagged benchmarks

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* engine: probe rotations, reaction moments and shell and beam element results; docs for shells and beams

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

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Co-authored-by: jyviko <2501648+jyviko@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

docs/elements.md

@@ -32,7 +32,7 @@ hex8/20/27 with first and second derivatives).
 |---|---|---|
 | Truss, 2-node (2D and 3D) | Axial members, cables (compression allowed) | done (rod2: implicit, in plane models with their two unknowns per node; explicit, corotational under nonlinear geometry; Abaqus T3D2 / T2D2, Nastran CROD, LS-DYNA truss beam (3D models)) |
 | Truss, 3-node (2D and 3D) | Curved axial members | done (rod3: implicit, plane models too; explicit with small displacements; Abaqus T3D3 / T2D3; no Nastran or LS-DYNA counterpart) |
-| Beam, 2-node Timoshenko, 3D | Frames, stiffeners; shear deformable | in progress (shells and beams) |
+| Beam, 2-node Timoshenko, 3D | Frames, stiffeners; shear deformable | done (exact interdependent interpolation, profile properties, orientation vector; `modelcraft_fem::beam`) |
 | Beam, 2-node Euler–Bernoulli (cubic), 2D and 3D | Slender frames; exact for end-loaded prismatic members | planned (1D elements II) |
 | Beam, 3-node Timoshenko (quadratic), 2D and 3D | Curved and stocky beams | planned (1D elements II) |
 | Beam, 2D (planar frames) | In-plane frames with 3 DOFs per node | planned (1D elements II) |
@@ -57,7 +57,7 @@ hex8/20/27 with first and second derivatives).
 
 | Element | Use | Status |
 |---|---|---|
-| Shell quad4 (MITC4), triangle (locking-free) | Thin and thick shells | in progress (shells and beams) |
+| Shell quad4 (MITC4), triangle (locking-free) | Thin and thick shells | done (flat MITC4 and MITC3 with a Hughes–Brezzi drilling rotation; `modelcraft_fem::shell`) |
 | Shell quad8 / tri6 | Curved thick shells | planned |
 | Continuum shells (8-node, on swept hex) | Thick-to-thin shells, composites through thickness | planned (M8) |
 | Membranes (tri3, quad4, quad8) | Fabrics, inflatables, skins without bending | planned |