fix(effects): clear Cut Away 100 opaque edges

The gradient antialias could leave a half-covered pixel where an opaque source reaches h = 1. Add an endpoint guard on CPU and GPU and cover an opaque edge in the regression test.

crates/gpu/src/shaders/fx_transition.wgsl

@@ -582,6 +582,12 @@ fn ftr_blob(@builtin(global_invocation_id) gid: vec3<u32>) {
         return;
     }
     let cut = P.f[0].x;
+    if (cut >= 1.0) {
+        // Endpoint guard: an opaque source can reach h = 1.0, where the gradient antialias
+        // would otherwise leave a half-covered pixel at Cut Away 100.
+        textureStore(out, p, vec4<f32>(0.0));
+        return;
+    }
     let hv = textureLoad(aux, p, 0).x;
     let g = ftr_grad(p);
     let grad = length(g);

crates/gpu/src/tests_fx_transition.rs

@@ -126,6 +126,7 @@ fn distort() {
         "ec.distort.ccblobbylize",
         &[("property", e(4)), ("softness", n(3.0)), ("cutAway", n(30.0)), ("lightType", e(1)), ("lightPosition", pt(20.0, 10.0))],
     );
+    effect_case("ec.distort.ccblobbylize", &[("cutAway", n(100.0))]);
     effect_case("ec.distort.ccblobbylize", &[("property", e(5)), ("metal", n(30.0)), ("roughness", n(0.2))]);
 }