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/engine/src/tests.rs

@@ -927,6 +927,7 @@ fn step_round_trip(s: &mut Session, tag: &str) -> solvecraft_kernel::StepImport
     let text = std::fs::read_to_string(&p).unwrap();
     let _ = std::fs::remove_file(&p);
     solvecraft_kernel::step_validate(&text).unwrap();
+    assert_eq!(solvecraft_kernel::step_orientation_errors(&text).unwrap(), Vec::<String>::new());
     assert!(text.contains("AP242_MANAGED_MODEL_BASED_3D_ENGINEERING_MIM_LF"));
     let imp = solvecraft_kernel::step_import(&text).unwrap();
     assert!(imp.warnings.is_empty(), "{:?}", imp.warnings);
@@ -945,7 +946,7 @@ fn sample_plate_step_round_trip() {
         let (ma, mb) = (solvecraft_kernel::measure(&a.body).unwrap(), solvecraft_kernel::measure(&b.body).unwrap());
         assert!(rel(mb.volume, ma.volume) < 1e-4, "{} vs {}", mb.volume, ma.volume);
         assert!(rel(mb.area, ma.area) < 1e-4, "{} vs {}", mb.area, ma.area);
-        assert_eq!(b.file_faces, a.body.face_count());
+        assert!(b.file_faces <= a.body.face_count() && b.file_faces >= ma.merged.faces.min(a.body.face_count()));
         assert_eq!(mb.merged.faces, ma.merged.faces);
     }
 }