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

crates/engine/src/cmd/forms.rs

@@ -820,6 +820,7 @@ pub const SAMPLE: &[Field] = &[choice(
         ("flow-past", "Flow past a bracket"),
         ("heated-bracket", "Heated bracket"),
         ("bracket-hex", "Hex-meshed bracket"),
+        ("crank-arm", "Bicycle crank arm"),
     ],
 )
 .req()