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/lib.rs

@@ -43,7 +43,7 @@ pub use polybool::planar_boolean;
 pub use polyhedron::{HalfSpace, convex_polyhedron, draft, offset_faces, shell};
 pub use sew::{EdgeSpec, FaceSpec, SurfSpec, sew};
 pub use step::step_export;
-pub use step_in::{ImportedBody, StepImport, StepNode, step_import, step_import_shared, step_validate};
+pub use step_in::{ImportedBody, StepImport, StepNode, step_import, step_import_shared, step_orientation_errors, step_validate};
 pub use step_out::{ExportBody, ExportProduct, StepHeader, step_export_bodies, step_export_products};
 pub use topo::{CylinderFace, cylinder_face_at};
 pub use topo::{TopoCounts, merged_topology, seam_flags};