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/geom.rs

@@ -133,7 +133,7 @@ pub(crate) fn frame(cx: &Ctx, id: u64) -> R<Frame> {
 }
 
 /// AXIS1_PLACEMENT: origin and unit axis.
-fn axis1(cx: &Ctx, id: u64) -> R<(mt::Point3, mt::Vector3)> {
+pub(crate) fn axis1(cx: &Ctx, id: u64) -> R<(mt::Point3, mt::Vector3)> {
     let e = entity(cx, id)?;
     let p = e.params();
     let o = point(cx, p.get(1).and_then(Param::as_ref_id).ok_or("axis without location")?)?;
@@ -875,8 +875,10 @@ pub(crate) fn surface(cx: &Ctx, id: u64, extent: &[mt::Point3], seam: &[mt::Poin
                 CurveGeo::Line { p, d } => revolved_line(*p, *d, o, axis, extent)?,
                 _ => bounded_curve(&g, extent)?,
             };
-            // Reversed sense: turn the other way round the axis.
-            Ok(revolved(profile, o, axis * sign))
+            // ISO 10303-42 parameterises a surface of revolution by the angle u and the profile
+            // v, so its normal is ∂S/∂angle × ∂S/∂profile: the opposite of truck's revolution
+            // (profile first). Fusion reads it this way. Reversed sense turns it back.
+            Ok(revolved(profile, o, -axis * sign))
         }
         "SURFACE_OF_LINEAR_EXTRUSION" => {
             let g = curve(cx, p.get(1).and_then(Param::as_ref_id).ok_or("extrusion without profile")?, depth + 1)?;