fix: use rounded-square SDF for cursor avoidance hole

Replace Chebyshev distance with a rounded box SDF for the cursor
avoidance shape. Eliminates diagonal artifacts where the sharp corners
of a pure square distance field intersected the ring geometry.

src/shader.wgsl

@@ -60,12 +60,12 @@ fn fs_main(@builtin(position) pos: vec4f) -> @location(0) vec4f {
         alpha = exp(-3.0 * t * t);
     }
 
-    // Cursor avoidance
-    let cursor_dist = distance(pos.xy, u.cursor_pos);
-    let avoid_start = u.cursor_avoid_radius;
-    let avoid_end = u.cursor_avoid_radius + u.cursor_avoid_falloff;
-    if cursor_dist < avoid_end {
-        let t_avoid = clamp((cursor_dist - avoid_start) / (avoid_end - avoid_start), 0.0, 1.0);
+    // Cursor avoidance — rounded square (box SDF with corner rounding)
+    let cursor_half = vec2f(u.cursor_avoid_radius);
+    let cursor_corner = u.cursor_avoid_radius * 0.3; // 30% rounding smooths the diagonals
+    let cursor_sd = rounded_rect_sdf(pos.xy - u.cursor_pos, cursor_half, cursor_corner);
+    if cursor_sd < u.cursor_avoid_falloff {
+        let t_avoid = clamp(cursor_sd / u.cursor_avoid_falloff, 0.0, 1.0);
         alpha *= t_avoid * t_avoid * (3.0 - 2.0 * t_avoid);
     }