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.BrandLogo.cs

@@ -97,6 +97,11 @@ public partial class MainViewModel
         Height = BrandLogoHeight,
     };
 
+    /// <summary>The staged scene's brand logo, exposed so the preview can draw it ABOVE the universal
+    /// blur (the compositor blits it after the blur group — the logo is never obscured).</summary>
+    public Source? StagedBrandLogo =>
+        StagedScene?.Elements.OfType<Source>().FirstOrDefault(s => s.IsBrandLogo);
+
     /// <summary>Exactly one brand logo per scene, placed directly above the background, carrying
     /// the current App Settings image.</summary>
     internal void EnsureBrandLogos()