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.

ViewModels/MainViewModel.Scenes.cs

@@ -37,6 +37,7 @@ public partial class MainViewModel
                 // binding that is never re-notified stays collapsed for the rest of the session
                 // (build 1082: the option vanished after the first layout load).
                 OnPropertyChanged(nameof(CanAddBlur));
+                OnPropertyChanged(nameof(StagedBrandLogo));
 
                 SelectedElement = null;
                 if (value != null)