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

docs/examples.md

@@ -112,6 +112,24 @@ peaks at 295 MPa at the upright's hole.
 solution of the unpartitioned part (0.5093 mm, `crates/testkit/tests/bracket_blocks.rs`), and the
 clamp carries the whole 1 kN.
 
+## Mirrored crank arm (`crank-mirror`)
+
+![Von Mises stress on the mirrored crank arm: one swept half and its mirror image, one mesh](media/examples/crank-mirror.png)
+
+The crank arm of `crank-arm`, built from a half. A datum plane on its symmetry plane (`Sym`,
+y = 0) partitions it (`Half`) into `CrankArm/a` and `CrankArm/b`; the half `CrankArm/b` is swept
+all-hex (hex20 of 3 mm, 3 330 elements) and `mesh.mirror` reflects it onto `CrankArm/a`, joined
+on the plane: 6 660 hex20 elements, 32 933 nodes, all valid (scaled Jacobian min 0.009, mean 0.91).
+The image's faces carry `CrankArm/a`'s names, so the sets pick the square bore and the pedal hole
+on both halves. Material, supports and load as `crank-arm`.
+
+**Look at:** the two halves' meshes, mirror images of each other across the arm's middle (Mesh
+workspace), and the datum plane `Sym` before solving.
+
+**Check:** σxx on the arm's sides at x = 29 mm is −62.43 and +62.43 MPa, equal and opposite as
+the halves are, against M/Z = 62.5 MPa (0.2 %); the pedal bore moves 0.75 mm down, as on the whole
+swept crank; the spindle carries the whole 1500 N.
+
 ## Bicycle crank arm (`crank-arm`)
 
 ![Von Mises stress on the all-hex crank arm, the arm root and the square bore carrying the most](media/examples/crank-arm.png)