Tests: a torus patch to thicken is cut from SMLib's one-face torus

SMLib makes a torus one face, so the face at (25, 0, 0) was the whole torus,
not a patch within half a turn each way. The patch is now the torus's piece
with x, y > 0 and z > 1, bounded by iso lines both ways (planes through the
axis, and z = 1 round the tube), and its thickened volume is the formula's
over the angles its mesh spans. (The sphere cap above it builds once SMLib's
offset takes an open edge's second trim point away from its first: SMLib
devgreen-kernel-geometry; waits for the pin.)

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

crates/kernel/src/surfaces.rs

@@ -1883,8 +1883,15 @@ mod tests {
         // A torus patch (R 20, r 5): between the tube radii r and r + 1 over its own angles, θ round
         // the tube and φ round the axis (read off its mesh): the volume is
         // Δφ·[R((r+1)² − r²)/2·Δθ + ((r+1)³ − r³)/3·(sin θ1 − sin θ0)].
-        let ring = torus(Vec3::ZERO, 20.0, 5.0).unwrap();
-        let face = copy_faces(&ring, &[Vec3::new(25.0, 0.0, 0.0)]).unwrap();
+        // SMLib makes a torus one face: the patch is the piece of it with x, y > 0 and z > 1
+        // (bounded by iso lines both ways: planes through the axis, and z = 1 round the tube).
+        let mut ring = torus(Vec3::ZERO, 20.0, 5.0).unwrap();
+        for (o, n) in [(Vec3::ZERO, Vec3::X), (Vec3::ZERO, Vec3::Y), (Vec3::new(0.0, 0.0, 1.0), Vec3::Z)] {
+            let pieces = crate::split_by_plane(&ring, &Plane::from_normal(o, n).unwrap()).unwrap();
+            ring = pieces.into_iter().find(|b| (measure(b).unwrap().bbox.min - o).dot(n) > -1e-6).unwrap();
+        }
+        let (c, s) = (std::f64::consts::FRAC_PI_4.cos(), std::f64::consts::FRAC_PI_4.sin());
+        let face = copy_faces(&ring, &[Vec3::new((20.0 + 5.0 * c) * c, (20.0 + 5.0 * c) * s, 5.0 * s)]).unwrap();
         let mesh = face.mesh(&crate::MeshTolerance::chord(0.01)).unwrap();
         let angles: Vec<(f64, f64)> = mesh.positions.iter().map(|p| (p.z.atan2((p.x * p.x + p.y * p.y).sqrt() - 20.0), p.y.atan2(p.x))).collect();
         let span = |f: &dyn Fn(&(f64, f64)) -> f64| {