Sample: crank-mirror, the crank arm meshed from a half and mirrored

file.sample crank-mirror partitions the crank arm on its symmetry plane, sweeps
one half all-hex (hex20) and mirrors it into one conforming mesh, then defines
the crank-arm model. Samples may now run preparing commands (datums,
partitions) before their script sees the bodies. Its test checks the halves
alike and joined, every element valid and the sets on both halves; the heavy
test solves it: sigma_xx at the root -62.43 / +62.43 MPa against M/Z = 62.5.

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

crates/engine/src/cmd/forms.rs

@@ -895,6 +895,7 @@ pub const SAMPLE: &[Field] = &[choice(
         ("heated-bracket", "Heated bracket"),
         ("bracket-hex", "Hex-meshed bracket"),
         ("crank-arm", "Bicycle crank arm"),
+        ("crank-mirror", "Mirrored crank arm"),
     ],
 )
 .req()