V210 issue fix + correct packing

internal/generator/wgpu_test.go

@@ -54,3 +54,37 @@ func TestWGPUSMPTEPattern(t *testing.T) {
 		})
 	}
 }
+
+func TestWGPUGeneratorUsesPaddedV210Rows(t *testing.T) {
+	const width, height = 100, 2
+	shader, err := kernels.Read("static/ebu75.wgsl")
+	if err != nil {
+		t.Fatalf("read shader: %v", err)
+	}
+	g, err := NewWGPUGenerator(width, height, string(shader))
+	if err != nil {
+		t.Fatalf("init: %v", err)
+	}
+	defer g.Close()
+
+	buf := make([]byte, V210FrameSize(width, height))
+	for i := range buf {
+		buf[i] = 0xff
+	}
+	if err := g.GenerateFrame(buf, 0); err != nil {
+		t.Fatalf("GenerateFrame: %v", err)
+	}
+
+	for y := 0; y < height; y++ {
+		gotY, gotCb, gotCr := sampleV210(buf, width, 0, y)
+		if gotY != 721 || gotCb != 512 || gotCr != 512 {
+			t.Fatalf("row %d first pixel = %d/%d/%d, want 721/512/512", y, gotY, gotCb, gotCr)
+		}
+		padding := buf[y*V210LineSize(width)+V210ActiveLineSize(width) : (y+1)*V210LineSize(width)]
+		for i, b := range padding {
+			if b != 0 {
+				t.Fatalf("row %d padding byte %d = %#x, want 0", y, i, b)
+			}
+		}
+	}
+}