Fix boss-bottom blends and merged arc-corner surfaces (#1294)

A fillet or chamfer on a cylindrical boss's bottom arc now builds with a closed display mesh: the seam-extension containment ray counts a shared curved-boundary endpoint once. Merging the split faces of a three-edge arc corner rebuilds the face on the original input face's full surface instead of a piece's restricted patch, and runs under the kernel's panic guard. Fixes #1262 and #1263.

Co-authored-by: Karl-VonRichter <20079846+Karl-VonRichter@users.noreply.github.com>

crates/kernel/src/blend.rs

@@ -231,6 +231,11 @@ pub(crate) fn extension_fits(face: &mt::Face, p: Vec3, q: Vec3, tol: f64) -> boo
             {
                 return false;
             }
+            // Curve endpoints may round to opposite sides of a ray that follows a
+            // straight boundary into the face. Treat points within its clearance as
+            // on the ray, so a shared endpoint contributes one crossing.
+            let ay = if ay.abs() <= tol { 0.0 } else { ay };
+            let by = if by.abs() <= tol { 0.0 } else { by };
             if (ay > 0.0) != (by > 0.0) {
                 let hit = ax - ay * (bx - ax) / (by - ay);
                 if hit > tol && hit <= len + tol {
@@ -2663,12 +2668,20 @@ mod tests {
             ),
             (Shape::Flat, 0.32, 0.8 / 3.0),
         ] {
-            let out = blend(&body, &[Vec3::new(52.0, 20.0, 0.0)], 0.8, shape, "blend").unwrap();
+            let picks = [Vec3::new(52.0, 20.0, 0.0)];
+            let out = match shape {
+                Shape::Round => fillet(&body, &picks, 0.8),
+                _ => chamfer(&body, &picks, 0.8),
+            }
+            .unwrap();
             assert!(out.validity().is_empty(), "{:?}", out.validity());
             // Pappus: the section turned half round the axis at its centroid's radius.
             let want = area * std::f64::consts::PI * (12.0 - c);
             let removed = v0 - vol(&out);
             assert!((removed - want).abs() < want * 2e-3, "{removed} vs {want}");
+            // The app's planned mesh must also close, with the same analytic removal.
+            let planned = crate::issue_851_tests::planned_volume(&body) - crate::issue_851_tests::planned_volume(&out);
+            assert!((planned - want).abs() < want * 2e-3, "display mesh: {planned} vs {want}");
         }
     }