fix(breadcrumb): the hover and focus wash follow the plate's squircle corners

The wash's rounded ends were cut on circular arcs while the run's plate is
drawn in the DE's corner family (layout::corner_shape). At corner_shape 4.5
the arc stood up to ~4 px inside the plate's corners at the top and bottom
rows, so the highlight did not cover the segment it marks. The ends now use
the same superellipse, |x|^n + |y|^n = r^n.

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

 src/widget/container/breadcrumb.rs | 53 ++++++++++++++++++++++++++++++--------
 1 file changed, 42 insertions(+), 11 deletions(-)

diff --git a/src/widget/container/breadcrumb.rs b/src/widget/container/breadcrumb.rs
index c229780..cee62de 100644
--- a/src/widget/container/breadcrumb.rs
+++ b/src/widget/container/breadcrumb.rs
@@ -289,6 +289,28 @@ impl Breadcrumb {
             .collect()
     }
 
+    /// How far the run's rounded end stands in from its upright edge at height
+    /// `yc`, for a band `y..y + h` with corner radius `r` — the wash's outer
+    /// ends follow the plate's silhouette by this. The corner is the DE's
+    /// family, `|x|^n + |y|^n = r^n` at exponent `n` (`layout::corner_shape`):
+    /// the plate is drawn at the nominal radius in that family, so a circular
+    /// arc here (as it was) cut inside a squircle plate's corners and left
+    /// them unwashed — some 4 px at the top and bottom rows at n = 4.5.
+    fn end_inset(r: f32, n: f32, y: f32, h: f32, yc: f32) -> f32 {
+        let dy = if yc < y + r {
+            r - (yc - y)
+        } else if yc > y + h - r {
+            yc - (y + h - r)
+        } else {
+            return 0.0;
+        };
+        if r <= 0.0 {
+            return 0.0;
+        }
+        let t = (dy / r).clamp(0.0, 1.0);
+        r - r * (1.0 - t.powf(n)).max(0.0).powf(1.0 / n)
+    }
+
     fn bg_color(&self) -> [f32; 4] {
         let c = crate::color::breadcrumb_bg_color();
         [c[0], c[1], c[2], self.network_opacity]
@@ -416,17 +438,8 @@ impl Paint for Breadcrumb {
                 let first = segs.first().map(|s| s.x) == Some(sx0);
                 let last = segs.last().map(|s| s.x + s.w) == Some(sx0 + sw);
                 let rr = crate::layout::dropdown_corner_radius().min(rh * 0.5);
-                // The rounded end's horizontal inset at height yc.
-                let arc = |yc: f32| -> f32 {
-                    let dy = if yc < ry + rr {
-                        rr - (yc - ry)
-                    } else if yc > ry + rh - rr {
-                        yc - (ry + rh - rr)
-                    } else {
-                        return 0.0;
-                    };
-                    rr - (rr * rr - dy * dy).max(0.0).sqrt()
-                };
+                let n = crate::layout::corner_shape();
+                let arc = |yc: f32| Self::end_inset(rr, n, ry, rh, yc);
                 let mut y = hy;
                 while y < hy + hh {
                     let bh = 1.0f32.min(hy + hh - y);
@@ -875,6 +888,24 @@ mod tests {
         }
     }
 
+    /// The wash's rounded ends are the plate's corner family: a circle at
+    /// n = 2, and at a squircle's exponent the corner stays nearer the square,
+    /// so the inset at the top row is smaller — what a circular arc got wrong.
+    #[test]
+    fn the_wash_ends_follow_the_plates_corner_family() {
+        let (r, y, h) = (10.0, 0.0, 24.0);
+        let top = 0.5;
+        let circle = Breadcrumb::end_inset(r, 2.0, y, h, top);
+        let dy: f32 = r - top;
+        assert!((circle - (r - (r * r - dy * dy).sqrt())).abs() < 1e-4);
+        let squircle = Breadcrumb::end_inset(r, 4.5, y, h, top);
+        assert!(squircle < circle - 2.0, "squircle {squircle} vs circle {circle}");
+        // Mid-height is the upright edge in either family, and the band's
+        // two ends mirror each other.
+        assert_eq!(Breadcrumb::end_inset(r, 4.5, y, h, h * 0.5), 0.0);
+        assert!((Breadcrumb::end_inset(r, 4.5, y, h, h - top) - squircle).abs() < 1e-4);
+    }
+
     #[test]
     fn path_controller_reachable_through_element() {
         let mut breadcrumb = Breadcrumb::new();