solidcraft-smlib: a revolution goes to SMLib as its NURBS form alone

A face on a placed revolution that carries its NURBS form (every STEP surface of
revolution, read since b6a3b31ff) was written as two NURBS blocks: the revolution's own
net, then the form, flagged FACE_NURBS_FORM on a NURBS face. SMLib reads one block per
face, so the flat arrays did not add up and it refused the body (SM_ERR_INVALID_INPUT),
in the main import and in the one-face sheets SMLib makes pcurves on. SMLib has no class
for a revolution: the form is the face, its range and trims already in its parameters,
so it now goes as that NURBS face with no form flag. A revolution the analytic pass
recognises (a plane, cylinder, cone, sphere, ring torus) is unchanged.

Test: surface_of_revolution_sense_follows_iso, which develop fails with the refusal,
passes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LUXeKMoxpM7Vuv8Rb6E3K2

crates/smlib/src/import.rs

@@ -185,8 +185,11 @@ fn laid_out(brep: &b::Brep, trusted: bool, analytic: bool, bare: &HashSet<(u32,
         a.i(flat::FACE_LOOP_COUNT).push(int(f.loops.len())?);
         a.i(flat::FACE_TRIM_TYPE).push(0);
         // A face with a NURBS form: what it is (its analytic entry), then the NURBS it is
-        // parameterised on, whose range and trims these are (FACE_NURBS_FORM).
+        // parameterised on, whose range and trims these are (FACE_NURBS_FORM). A revolution
+        // (SMLib has no class for one) is that form alone, a NURBS face.
+        let form_is_face = f.nurbs.is_some() && matches!(f.surface, b::Surface::PlacedRevolution { .. });
         let spline = match &f.nurbs {
+            Some(n) if form_is_face => surface(&mut a, n, f.domain, analytic)?,
             Some(n) => {
                 surface(&mut a, &f.surface, f.domain, analytic)?;
                 nurbs_block(&mut a, n)?;
@@ -195,7 +198,7 @@ fn laid_out(brep: &b::Brep, trusted: bool, analytic: bool, bare: &HashSet<(u32,
             None => surface(&mut a, &f.surface, f.domain, analytic)?,
         };
         if any_form {
-            a.i(flat::FACE_NURBS_FORM).push(i64::from(f.nurbs.is_some()));
+            a.i(flat::FACE_NURBS_FORM).push(i64::from(f.nurbs.is_some() && !form_is_face));
         }
         for (li, l) in f.loops.iter().enumerate() {
             match l {