feat: truncate the plate roll's descent at 0.8 of the quadrant

The superellipse quadrant packs nearly all of its drop into the outer
third of the roll, so widget plates ended on a hard near-vertical
dropoff line at the silhouette. roll_slope now samples the profile at
f * ROLL_CUT: the rim shades as a slab cut off partway down the
descent, ending on a bounded slope. Carve walls are untouched.

Co-Authored-By: Claude Fable 5 <[email protected]>

 src/vk/shader2d.wgsl | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/src/vk/shader2d.wgsl b/src/vk/shader2d.wgsl
index dad5a8c..2ca4a0c 100644
--- a/src/vk/shader2d.wgsl
+++ b/src/vk/shader2d.wgsl
@@ -190,13 +190,21 @@ fn roll_spec(sv: vec2f) -> f32 {
 // join, so the roll's shading fades into the face instead of ending on a line.
 // The slope has the closed form (f/h)^(n-1), which IS the circular formula at
 // n = 2 — the same one-exponent generalization as the plan corners.
+// The descent is truncated at ROLL_CUT of the quadrant: the roll shades as if
+// the slab's rim were cut off partway down, so the profile ends on a bounded
+// slope instead of plunging vertical at the silhouette (the full quadrant put
+// nearly all of its drop in the outer third of the roll, reading as a hard
+// dropoff line at the very edge).
+const ROLL_CUT: f32 = 0.8;
+
 fn roll_slope(f: f32) -> f32 {
     let shape = rrect_clip.rect1.w;
+    let fc = f * ROLL_CUT;
     if (shape > 2.001) {
-        let h = pow(max(1.0 - pow(f, shape), 1e-4), 1.0 / shape);
-        return pow(f / h, shape - 1.0);
+        let h = pow(max(1.0 - pow(fc, shape), 1e-4), 1.0 / shape);
+        return pow(fc / h, shape - 1.0);
     }
-    return f / sqrt(max(1.0 - f * f, 1e-4));
+    return fc / sqrt(max(1.0 - fc * fc, 1e-4));
 }
 
 // Slope of a carve's transition profile (0 on the surrounding plateau → 1 on