feat(blur): universal — the blur covers every layer, webcam included

The creator found the use case that reframes the feature: a moment of privacy while the webcam is
on. A blur that can only cover the desktop cannot do that.

- SceneCompositor applies the blur group AFTER every element is composited (still before the
  flash / social bar / ticker / zoom overlays), so it covers webcam, chat, alerts — anything. Its
  position in the layer list no longer affects the output, so it is added at the top and the
  reorder clamp is gone.
- Supersedes the earlier "blur sits below the brand" rule: a universal blur does obscure the logo
  when it covers it. Called out in frame-pipeline.md so it is not re-litigated.
- Preview mirrors it exactly: the blur host moved OUT of CanvasGrid to a sibling above it, so the
  VisualBrush samples every layer (and cannot sample itself). It used to sample only the backdrop,
  which is why a circle over the webcam would have shown sharp pixels in the editor and blurred
  ones in the stream.

Controls/PreviewPane.xaml

@@ -20,6 +20,11 @@
                           PreviewMouseRightButtonUp="Preview_MouseRightButtonUp">
                         <Viewbox Stretch="Uniform">
                             <Canvas x:Name="PreviewCanvas" Width="1920" Height="1080" ClipToBounds="True">
+                                <!-- Privacy blur preview (2026-11-01): the blur is UNIVERSAL — it covers
+                                     every layer beneath it, so this host sits ABOVE the whole CanvasGrid
+                                     rather than inside it. Each child samples CanvasGrid (all layers, no
+                                     recursion because this canvas is a sibling) at the blur's rect and
+                                     carries a BlurEffect at the output's strength. -->
                             <Grid x:Name="CanvasGrid" Width="1920" Height="1080" ClipToBounds="True" Background="Transparent">
                                 <Grid.RenderTransform>
                                     <TransformGroup>
@@ -71,15 +76,6 @@
                                            Visibility="{Binding BackgroundVisible, Converter={StaticResource BoolToVis}}"/>
                                     <Image Source="{Binding ActiveBackgroundImage}" Stretch="UniformToFill"/>
                                 </Grid>
-                                <!-- Privacy blur preview (2026-11-01): whatever is BENEATH a blur layer
-                                     is blurred, right here in the editor — not a hatched placeholder. Each
-                                     child is a rectangle/ellipse filled with a VisualBrush of the whole
-                                     BackdropLayer (the desktop + background art), clipped to the blur's
-                                     shape and blurred at the same strength the output uses. Hosted AFTER
-                                     the backdrop and BEFORE the element layers, which is the model's own
-                                     z-rule: the blur sits above the desktop and below the brand. -->
-                                <Canvas x:Name="BlurPreviewHost" Width="1920" Height="1080" IsHitTestVisible="False"/>
-
                                 <Image x:Name="TransitionOverlay" Width="1920" Height="1080" Stretch="Fill"
                                        IsHitTestVisible="False" Visibility="Collapsed"/>
                                 <Canvas x:Name="OverlayCanvas" Width="1920" Height="1080">
@@ -474,6 +470,7 @@
                                     <VisualBrush x:Name="ZoomVisualBrush" ViewboxUnits="Absolute" Stretch="Fill"/>
                                 </Grid.Background>
                             </Grid>
+                                <Canvas x:Name="BlurPreviewHost" Width="1920" Height="1080" IsHitTestVisible="False"/>
                             </Canvas>
                         </Viewbox>
                         <Border HorizontalAlignment="Left" VerticalAlignment="Top" Margin="8,8,0,0"

Controls/PreviewPane.xaml.cs

@@ -740,11 +740,11 @@ public partial class PreviewPane : UserControl
     }
 
     // ─── Privacy blur preview (2026-11-01) ───
-    // The editor has to show the blur it is about to bake into the output: "whatever is beneath the
-    // blur layer is blurred". Each blur becomes a rectangle/ellipse filled with a VisualBrush of the
-    // whole BackdropLayer (desktop + background art) sampled at that rect, with a BlurEffect at the
-    // same strength the compositor uses. The host canvas sits between the backdrop and the element
-    // layers, which is the model's own z-rule.
+    // The editor has to show the blur it is about to bake into the output, and the blur is
+    // UNIVERSAL: it covers every layer, webcam and chat included. Each blur becomes a
+    // rectangle/ellipse filled with a VisualBrush of the whole CanvasGrid sampled at that rect,
+    // with a BlurEffect at the same strength the compositor uses. The host canvas is a SIBLING
+    // above CanvasGrid, which is what stops the brush from sampling itself.
     private MainViewModel? _blurVm;
     private Scene? _blurScene;
 
@@ -811,7 +811,7 @@ public partial class PreviewPane : UserControl
             {
                 Width = blur.Width,
                 Height = blur.Height,
-                Fill = new VisualBrush(BackdropLayer)
+                Fill = new VisualBrush(CanvasGrid)
                 {
                     Viewbox = new Rect(blur.X, blur.Y, blur.Width, blur.Height),
                     ViewboxUnits = BrushMappingMode.Absolute,