engine: crank-arm sample, a bicycle crank swept all-hex and solved

file.sample crank-arm: a 170 mm 6061-T6 crank built in SolidCraft
(crates/engine/samples/crank-arm.json), swept along z by mesh.hex.generate
as 7274 hex20 elements of 3 mm (scaled Jacobian min 0.17). The hex variant
drops the pocket floor fillet and boss chamfers, and its root arcs meet the
bosses 20 degrees off tangent so the arm's top face has no zero-angle cusp.
The square bore is fixed and 1500 N acts on the pedal hole; the faces are
picked by their surfaces, not their names. Its heavy test matches the root
bending stress to M/Z = 62.6 MPa within 3 % (0.3 % in practice).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

README.md

@@ -102,8 +102,8 @@ against a closed form or a reference solution ([docs/examples.md](docs/examples.
 |---|---|---|
 | ![Drop test](docs/media/examples/drop-test.png) | ![Rubber seal](docs/media/examples/rubber-seal.png) | ![Heated bracket](docs/media/examples/heated-bracket.png) |
 | **Drop test** (`drop-test`): an aluminium bracket dropped 2 m onto a tilted floor; explicit dynamics, the strike's stress waves | **Rubber seal** (`rubber-seal`): a Mooney–Rivlin gasket ring squeezed 25 %; nonlinear static, large strain | **Heated bracket** (`heated-bracket`): a clamped steel bracket heated at one end; steady heat and thermal stress |
-| ![Hex-meshed bracket](docs/media/examples/bracket-hex.png) | *No picture yet: the viewport does not draw flow results (M12).* | |
-| **Hex-meshed bracket** (`bracket-hex`): the bracket split into 30 cells, meshed all-hex (hex20), von Mises | **Flow past a bracket** (`flow-past`): air past an L-bracket on the floor; lattice Boltzmann flow, the drag and the force on each face | |
+| ![Hex-meshed bracket](docs/media/examples/bracket-hex.png) | *No picture yet: the viewport does not draw flow results (M12).* | ![Bicycle crank arm](docs/media/examples/crank-arm.png) |
+| **Hex-meshed bracket** (`bracket-hex`): the bracket split into 30 cells, meshed all-hex (hex20), von Mises | **Flow past a bracket** (`flow-past`): air past an L-bracket on the floor; lattice Boltzmann flow, the drag and the force on each face | **Bicycle crank arm** (`crank-arm`): an aluminium crank swept all-hex (hex20), a rider's weight on the pedal; root bending within 0.3 % of M/Z |
 
 ## Build