fix(vk): pin superellipse corner samples to the axis at quadrant ends

f32 sin/cos are not exactly zero at a quadrant end (sin(PI) is -8.7e-8),
and superellipse_pt raised that noise to a fractional power, which
magnifies it by orders of magnitude: at corner_shape 4.5 it is 7e-4. On
the desktop grid's 138 px cell corners that put each fan's tangent
vertex 0.1 px short of the body quad's edge, so the fan's edge tilted
away from the quad's and the row of pixel centres between them belonged
to neither: a stray gap-coloured line 32 px long at every cell's left
edge, one radius below the top, and a lone gap pixel on the bottom row
where the arc met the body. The compositor's fallback lattice (scenefx
rects) was clean, which is what singled out the client's tessellation.

Snap a component within 1e-6 of 0 or ±1 to the exact value; a real
sample never comes within that of an axis except at the ends themselves.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

 src/backend/window_runner.rs | 24 +++++++++++++++++++++---
 1 file changed, 21 insertions(+), 3 deletions(-)

diff --git a/src/backend/window_runner.rs b/src/backend/window_runner.rs
index 7140f24..b7d89e5 100644
--- a/src/backend/window_runner.rs
+++ b/src/backend/window_runner.rs
@@ -918,12 +918,30 @@ pub fn rounded_rect_vertices_corners(
 /// n = 2; higher `corner_shape` exponents give the DE's continuous-curvature
 /// corners, so widget silhouettes follow the same corner family as the
 /// SDF-lit plates. `e` is 2/n, hoisted by callers. Tangent points at the
-/// quadrant ends are unchanged, so fans still tile exactly against the body
-/// rects and edge strips.
+/// quadrant ends are pinned to the exact axis points (see below), so fans
+/// tile exactly against the body rects and edge strips.
 #[inline]
 fn superellipse_pt(theta: f32, e: f32) -> (f32, f32) {
     let (s, c) = theta.sin_cos();
-    (c.signum() * c.abs().powf(e), s.signum() * s.abs().powf(e))
+    // f32 sin/cos are not exactly 0 at a quadrant end (sin(PI) is -8.7e-8),
+    // and the fractional power magnifies that noise by orders of magnitude:
+    // at corner_shape 4.5 it is 7e-4, which on the desktop grid's 138 px
+    // cell corners put the fan's tangent vertex 0.1 px short of the body
+    // quad's edge. The fan's edge then tilts away from the quad's, and the
+    // row of pixel centres between them is covered by neither: a stray
+    // gap-coloured line 32 px long at every cell's left edge, and a lone
+    // gap pixel on the bottom row where the arc meets the body. Snap the
+    // ends to the exact axis points so fans tile against the rects.
+    let axis = |v: f32| -> f32 {
+        if v.abs() < 1e-6 {
+            0.0
+        } else if v.abs() > 1.0 - 1e-6 {
+            v.signum()
+        } else {
+            v.signum() * v.abs().powf(e)
+        }
+    };
+    (axis(c), axis(s))
 }
 
 /// Feathered glow ([`Prim::Glow`]): the rounded rect's interior fills at the