Import rig: the horn torus quarter written to STEP and read back, as a case

The round trip's volume moves from the STEP unit test to an import rig case
(616.850, Pappus); the unit test keeps the writer's logic, the degenerate torus
written as one B-spline face and no toroidal surface or surface of revolution. The
case fails today where the revolve does (KNOWN_FAILING).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/kernel/src/step_in/tests.rs

@@ -1127,7 +1127,8 @@ fn surface_of_revolution_sense_follows_iso() {
 
 /// A tube revolved about a line it touches (a horn torus, as a tight sweep makes) is written as
 /// one rational B-spline face whose loop passes the pole twice, with no zero-length edge there:
-/// no TOROIDAL_SURFACE (it would be degenerate) and no surface of revolution.
+/// no TOROIDAL_SURFACE (it would be degenerate) and no surface of revolution. (Its volume, built
+/// and read back, is the rig's import/cov-step-horn-torus-quarter-round-trip.)
 #[test]
 fn horn_torus_written_as_one_spline_face() {
     let prof = Region2 { outer: Loop2::circle(Vec2::new(5.0, 0.0), 5.0), holes: vec![] };
@@ -1135,9 +1136,6 @@ fn horn_torus_written_as_one_spline_face() {
     let text = step_export(&[&b], "t").unwrap();
     assert!(!text.contains("TOROIDAL_SURFACE") && !text.contains("SURFACE_OF_REVOLUTION"));
     assert_eq!(text.matches("ADVANCED_FACE(").count(), 3, "{text}");
-    let imp = round_trip(&b);
-    let v = std::f64::consts::PI * 25.0 * std::f64::consts::TAU * 5.0 / 4.0;
-    assert!(rel(measure(&imp.bodies[0].body).unwrap().volume, v) < 1e-3);
 }
 
 /// How a torus-capped cylinder ([`torus_capped_cylinder`]) ends.

tests/kernel-cases/import/KNOWN_FAILING

@@ -2,3 +2,4 @@
 # rig gate (.github/workflows/kernel-rig.yml) fails on a case that fails and is not listed here,
 # and on a listed case that passes: a fix removes its line in the same pull request.
 # Baseline: develop 5ada4fc8a with SMLib 90aabff (the run on 60b1c6a, aarch64 Linux).
+cov-step-horn-torus-quarter-round-trip SMLib revolve status 1001 on a profile touching its axis (the part is never built)

tests/kernel-cases/import/cov-step-horn-torus-quarter-round-trip/expected.json

@@ -0,0 +1,8 @@
+{
+ "sources": [],
+ "what": "a quarter horn torus (a circle r5 touching its axis, revolved 90 deg) written to STEP and read back: the same solid",
+ "source": "analytic",
+ "method": "the disc r5 about (5, 0) on XZ touches the Z axis at the origin; Pappus, its centroid 5 from the axis: V = pi 25 x 2 pi 5 / 4 = 616.850275; read back into a new design, the same",
+ "body_count": 1,
+ "volume_mm3": 616.8502750680849
+}

tests/kernel-cases/import/cov-step-horn-torus-quarter-round-trip/recipe.json

@@ -0,0 +1,56 @@
+{
+ "commands": [
+  {
+   "command": "sketch.create",
+   "params": {
+    "plane": "XZ",
+    "name": "Tube"
+   }
+  },
+  {
+   "command": "sketch.circle.center",
+   "params": {
+    "center": [
+     5,
+     0
+    ],
+    "radius": 5
+   }
+  },
+  {
+   "command": "sketch.finish",
+   "params": {}
+  },
+  {
+   "command": "solid.revolve",
+   "params": {
+    "sketch": "Tube",
+    "axis": "Z",
+    "angle": "90 deg",
+    "body_name": "Horn"
+   }
+  },
+  {
+   "command": "file.export",
+   "params": {
+    "path": "$OUT/kernel-case-horn-torus.step",
+    "format": "step",
+    "bodies": [
+     "Horn"
+    ]
+   }
+  },
+  {
+   "command": "file.new",
+   "params": {
+    "name": "Reimported"
+   }
+  },
+  {
+   "command": "file.insert_step",
+   "params": {
+    "path": "$OUT/kernel-case-horn-torus.step"
+   }
+  }
+ ]
+}