fix: report every fillet refusal of a radius too big as too large (#477)

`fit: reduce` acts only on refusals marked too large. Three kinds weren't:
the corner build's contact lines leaving their faces or running backwards,
and a curved chain's contact reaching past the end of the seam it trims. Two
more were hidden: a later edge's too-large refusal lost to an earlier edge's
other error, and the corner build's lost to the local blend's in `fillet`.
The messages are unchanged.

Refs #293

Co-authored-by: Claude Opus 5.5 <[email protected]>

crates/kernel/src/tests.rs

@@ -356,6 +356,29 @@ fn fillet_max_finds_the_largest_radius_that_works() {
     assert!(fillet_max(&b, &[Vec3::new(500.0, 0.0, 5.0)], 1.0).is_none());
 }
 
+/// A corner's rounds too large for its faces are refused as too large, so a fit can reduce
+/// them, also where only the corner build finds it (the edge-by-edge blend failing otherwise).
+#[test]
+fn corner_rounds_larger_than_their_faces_are_too_large() {
+    let (l, eps) = (20.0, 1e-9);
+    let b = box_solid(Vec3::ZERO, Vec3::new(l, l, l)).unwrap();
+    let corner = [Vec3::new(10.0, 0.0, 0.0), Vec3::new(0.0, 10.0, 0.0), Vec3::new(0.0, 0.0, 10.0)];
+    // As wide as the faces: the first edge's round uses up its neighbour, then the edge-by-edge
+    // blend can't find it; the corner build is what says the radius is too large.
+    let e = fillet(&b, &corner, l).unwrap_err();
+    assert!(matches!(e, KernelError::TooLarge(_)), "{e}");
+    let (r, body) = fillet_max(&b, &corner, l).unwrap();
+    assert!((r - 19.99).abs() < eps, "{r}");
+    assert!(body.validity().is_empty(), "{:?}", body.validity());
+    // Three rounds along the edges past the corner cube, and the cube less its sphere octant.
+    let removed = 3.0 * (1.0 - PI / 4.0) * r * r * (l - r) + r * r * r * (1.0 - PI / 6.0);
+    assert!(rel(measure(&body).unwrap().volume, l * l * l - removed) < 2e-4);
+    // Different radii per edge, one wider than its faces.
+    let sel: Vec<(Vec3, f64)> = corner.iter().copied().zip([21.0, 5.0, 5.0]).collect();
+    let e = crate::blend_corner::fillet_radii(&b, &sel).unwrap_err();
+    assert!(matches!(e, KernelError::TooLarge(_)), "{e}");
+}
+
 #[test]
 fn tapered_extrudes() {
     // 40 x 40 square, 30 up, 10° outward: a frustum.