Make layout blur, opacity, and watermark position apply in rendered videos.

Still-image encodes ignored boxblur and 1s black fills; switch to gblur plus full-length black frames, and honor brand watermark position.

scripts/layout.test.ts

@@ -26,7 +26,7 @@ function testEnumsAndClamp() {
   assert.equal(blurToBoxblur(0), null);
   const mid = blurToBoxblur(50);
   assert.ok(mid && mid.radius >= 1 && mid.power >= 1);
-  assert.ok(boxblurFilterSegment(60).includes("boxblur="));
+  assert.ok(boxblurFilterSegment(60).includes("gblur="));
   assert.equal(boxblurFilterSegment(0), "");
 }
 
@@ -197,10 +197,21 @@ function testGeometryAndFilter() {
     blurOpacity: 100,
   });
   assert.ok(classicBlur.includes("[0:v]split=2"));
+  assert.ok(classicBlur.includes("gblur=") || classicBlur.includes("[blurred]"));
   assert.ok(classicBlur.includes("overlay=(W-w)/2:(H-h)/2"));
   assert.equal(classicBlur.includes("pad="), false);
   assert.equal(classicBlur.includes("drawtext="), false);
   assert.ok(classicBlur.endsWith("[laid]"));
+
+  const classicOpacity = buildClassicBlurFillFilterComplex({
+    width: 1280,
+    height: 720,
+    blurAmount: 40,
+    blurOpacity: 50,
+  });
+  assert.ok(classicOpacity.includes("d=86400"));
+  assert.equal(classicOpacity.includes("shortest=1"), false);
+  assert.ok(classicOpacity.includes("blend="));
 }
 
 testEnumsAndClamp();