STEP export: pieces of a plane or extruded cylinder written as one face

Fusion round trip WARNs on face counts:
- 48: the pulley's annular end faces arrived as 240° and 120° sectors.
- 54: the side faces and holes of a body glued to its mirror image came
  in halves.

Changes:
- Planes merge: PLANE faces and flat B-spline sheets, as well as revolved
  planes. A plane keeps several loops (an annulus, a face with holes), no
  seam. Flat B-spline sheets are written as PLANE.
- Pieces of an extruded cylinder (B-spline sheets) merge when their edges
  are circles square to the axis. The merged face is written as a
  CYLINDRICAL_SURFACE, with its parameter seam on the face's seam edge.
- A full turn's loop is built explicitly: ring, seam, other ring (or the
  apex on the axis), seam back. Before, it depended on edge order, and
  after 81c85b5 the pulley's last pieces no longer merged.
- The read-back check moved to the whole file. If the merged file reads
  with warnings and the unmerged one does not, the unmerged one is
  written.

Results: 48 has 12 faces (Fusion 12), 54 has 12 (Fusion 12). Oracle
results are unchanged (73 PASS) and the STEP corpus passes 76/76. New
test: a plate with a hole glued to its mirror exports 8 faces, all
planes and cylinders, and round-trips.

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

@@ -692,7 +692,7 @@ fn revolved_oriented(curve: mt::Curve, o: mt::Point3, axis: mt::Vector3, u: f64,
     revolved(curve, o, -axis)
 }
 
-fn bspline_surface(cx: &Ctx, e: &Entity) -> R<mt::Surface> {
+pub(crate) fn bspline_surface(cx: &Ctx, e: &Entity) -> R<mt::Surface> {
     let (base, knots, kind): (Vec<Param>, Option<[Option<&Param>; 4]>, &str) = if e.is_complex() {
         let b = e.record("B_SPLINE_SURFACE").ok_or("complex surface without B_SPLINE_SURFACE")?;
         let mut base = vec![Param::Str(String::new())];