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

README.md

@@ -30,7 +30,7 @@ storytold "craft" family.
   deleted.
 - **Parameters** with unit-aware expressions (`2 * width + 5 mm`, `angle / 2`).
 - **Files**: designs as JSON (`.solvecraft`), export to STEP (AP242 with body names, colours and
-  components as assemblies), 3MF, STL (binary/ASCII) and OBJ;
+  components as assemblies; revolved faces as native planes, cylinders, cones, spheres and tori), 3MF, STL (binary/ASCII) and OBJ;
   3MF and STL import as mesh bodies (they render, measure, move and export; solid features need
   B-rep bodies);
   STEP import (AP203/AP214/AP242 solids, assemblies, units, names, colours) as a base feature