STEP export: Fusion round-trip fixes (sense, analytic surfaces, horn torus)

- Surfaces of revolution follow ISO 10303-42 strictly in reader and writer: the
  normal is d/d(angle) x d/d(profile), the opposite of truck's revolution, as
  Fusion reads it (fixes Fusion inverting fillet sphere corners and the dimple).
  step_orientation_errors flags faces whose loop runs clockwise round their
  normal; engine, oracle and kernel round trips assert it.
- Revolved faces with analytic profiles are written as native PLANE,
  CYLINDRICAL/CONICAL/SPHERICAL/TOROIDAL surfaces, and the pieces truck splits
  a revolution into are merged back into one face per surface (with a seam
  edge for full turns), so face counts match other systems.
- A horn torus (tight sweep: tube touching its axis) is written as one rational
  B-spline face whose loop passes the pole twice, with truck's zero-length pole
  edges collapsed, instead of a degenerate surface of revolution; the reader
  starts such figure-eight loops off the pole (Fusion's own part 50 now imports
  with the right volume).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AS5Tn3xieewUSg3uxrMVdL

crates/kernel/src/step_in/brep.rs

@@ -255,7 +255,22 @@ impl Builder {
                     })
                     .collect()
             }
-            _ => wires,
+            // A loop passing a vertex twice (a figure eight through a pole, as written for a
+            // torus whose tube touches its axis) starts at a vertex it passes once.
+            _ => wires
+                .into_iter()
+                .map(|w| {
+                    let edges: Vec<mt::Edge> = w.edge_iter().cloned().collect();
+                    let passes = |e: &mt::Edge| edges.iter().filter(|x| x.front().id() == e.front().id()).count();
+                    match edges.first().map(passes) {
+                        Some(n) if n > 1 => {
+                            let start = edges.iter().position(|e| passes(e) == 1).unwrap_or(0);
+                            edges.iter().skip(start).chain(edges.iter().take(start)).cloned().collect::<Vec<_>>().into()
+                        }
+                        _ => w,
+                    }
+                })
+                .collect(),
         };
         match mt::Face::try_new(wires.clone(), surface.clone()) {
             Ok(f) => Ok(f),