fix: bevel-profile LUT tapers to zero at the band's ends — faces faceted

The analytic carve profile happens to have zero slope at v=0 and v=1, so
the flat surfaces on either side of a wall (which the shader maps to the
saturated ends of v) stayed flat for free. The LUT's midpoint samples are
nonzero at the ends, so every face pixel inherited the endpoint slope and
the rounded-rect SDF gradient's nearest-edge regions faceted each face
into visible triangles (per-tile diagonals; large wedges on the plates).

The curve describes ONLY the wall band — taper the sampled slope to
exactly zero over the outer ~1.5 samples at both ends. Sharp-ended curves
(a linear chamfer) keep their crease; faces stay flat.

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

 src/vk/shader2d.wgsl | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/src/vk/shader2d.wgsl b/src/vk/shader2d.wgsl
index fefe832..f8bb32f 100644
--- a/src/vk/shader2d.wgsl
+++ b/src/vk/shader2d.wgsl
@@ -210,14 +210,21 @@ fn roll_slope(f: f32) -> f32 {
 fn carve_slope(v: f32) -> f32 {
     if (window_info.profile_meta.x > 0.5) {
         let n = window_info.profile_meta.y;
+        let vc = clamp(v, 0.0, 1.0);
         // Samples sit at v = (i + 0.5) / n; lerp between the two neighbors.
-        let x = clamp(clamp(v, 0.0, 1.0) * n - 0.5, 0.0, n - 1.0);
+        let x = clamp(vc * n - 0.5, 0.0, n - 1.0);
         let i0 = u32(floor(x));
         let i1 = min(i0 + 1u, u32(n) - 1u);
         let fr = x - floor(x);
         let s0 = window_info.profile[i0 >> 2u][i0 & 3u];
         let s1 = window_info.profile[i1 >> 2u][i1 & 3u];
-        return mix(s0, s1, fr);
+        // The curve describes ONLY the wall band; the surfaces on either side
+        // are flat by definition, so taper to exactly zero at both ends
+        // (~1.5 samples). Without this every face pixel (v saturates at 1
+        // inside a feature) inherits the endpoint slope, and the SDF
+        // gradient's nearest-edge regions facet the face into triangles.
+        let win = clamp(min(vc, 1.0 - vc) * n * 0.667, 0.0, 1.0);
+        return mix(s0, s1, fr) * win;
     }
     if (rrect_clip.rect1.w > 2.001) {
         let w = v * (1.0 - v);