feat(blur): the brand logo cannot be blurred

The blur is universal, so it would swallow the logo along with everything else. The compositor
now defers every visible brand logo: skipped in both element loops, blitted immediately AFTER
the blur group, before the flash/social/ticker overlays. One logo per scene, so it is cheap.

Preview mirrors it: the staged scene's brand logo is drawn above the blur host (StagedBrandLogo),
so what you see in the editor is what the encoder gets.

Test: a blur covering the logo's area leaves the logo's pixels exactly white while the coloured
background beneath it blends.

Controls/PreviewPane.xaml

@@ -471,6 +471,13 @@
                                 </Grid.Background>
                             </Grid>
                                 <Canvas x:Name="BlurPreviewHost" Width="1920" Height="1080" IsHitTestVisible="False"/>
+                                <!-- The brand logo rides ABOVE the blur host, mirroring the compositor:
+                                     a privacy blur can never swallow it. -->
+                                <Image Canvas.Left="{Binding StagedBrandLogo.X}" Canvas.Top="{Binding StagedBrandLogo.Y}"
+                                       Width="{Binding StagedBrandLogo.Width}" Height="{Binding StagedBrandLogo.Height}"
+                                       Source="{Binding StagedBrandLogo.DisplaySource}" Stretch="Fill"
+                                       IsHitTestVisible="False"
+                                       Visibility="{Binding StagedBrandLogo, Converter={StaticResource NotNullToVis}}"/>
                             </Canvas>
                         </Viewbox>
                         <Border HorizontalAlignment="Left" VerticalAlignment="Top" Margin="8,8,0,0"