SMLib tests: a round larger than the cube is cut as if its faces went on, at its exact volume

At the 7e917f0 pin SMLib builds the r50 round of a 20 cube's edge that
hostile_inputs_are_errors expected to fail: a convex blend that overflows its
faces' far edges is built by subtraction (blend on the faces extended, then cut),
as the chamfer past the faces two lines below already is. The body is exact: the
two ends, the two far faces and the round (5 faces, 9 edges, 6 vertices), volume
20 (F(40) - F(30) - 300) = 1094.85273 with F(u) = (u sqrt(2500 - u^2) +
2500 asin(u / 50)) / 2, the cube's section inside the r50 circle about
y = z = -30. The test checks that answer instead of an error.

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

crates/smlib/src/ops_tests.rs

@@ -206,11 +206,15 @@ fn hostile_inputs_are_errors() {
     for t in [0.0, f64::NAN, -1e7] {
         assert_eq!(c.shell(&[[10.0, 10.0, 20.0]], 0.1, t).unwrap_err(), Error::Argument, "thickness {t}");
     }
-    // Sizes SMLib cannot build fail as errors, whatever SMLib says.
-    match c.fillet(&mid, 0.1, 50.0) {
-        Ok(b) => panic!("radius larger than the cube: SMLib built a body of volume {:?}, counts {:?}", b.volume(ACC), b.counts()),
-        Err(e) => eprintln!("radius larger than the cube: {e:?}"),
-    }
+    // A round larger than the cube overflows its faces' far edges, convex, so it is cut as if the
+    // faces went on: the r50 cylinder about y = z = -30 uses up both faces of the edge and leaves
+    // the part of the cube's 20 x 20 section inside it, from (0, 10) to (10, 0), along x: 20 x
+    // that area, 20 (F(40) - F(30) - 300) with F(u) = (u sqrt(2500 - u^2) + 2500 asin(u / 50)) / 2.
+    let over = c.fillet(&mid, 0.1, 50.0).unwrap();
+    let f = |u: f64| 0.5 * (u * (2500.0 - u * u).sqrt() + 2500.0 * (u / 50.0).asin());
+    let want = 20.0 * (f(40.0) - f(30.0) - 300.0);
+    assert!((volume(&over) - want).abs() < 1e-6 * want, "radius larger than the cube: {} vs {want}", volume(&over));
+    assert_eq!(over.counts().unwrap(), [5, 9, 6], "the two ends, the two far faces and the round");
     // A chamfer past the faces runs over their far edges, cliffs: it is cut as if the faces went
     // on (the plane y + z = 10 through the setbacks 30 back from the edge y = z = 20), leaving the
     // triangle y + z <= 10 of the cube's section along x: 50 x 20.