Zoom: anchor the capture, magnify the full composite, draw on top (Unit: zoom rework per creator correction)

Creator corrected the "loupe" zoom on 2026-10-08: the click anchors a fixed
capture region (box/2 centred on the click) and the box is a live 2x view of
that region — dragging the box never changes its content; it magnifies the
fully-composited frame (every layer, incl. webcam) and draws LAST (n+1).
Rectangle shape is 4:3 ("more of a 640x480 aspect").

- MainViewModel.Zoom.cs rewritten: ActivateZoom anchors CaptureX/CaptureY
  (independently clamped); UpdateZoomMove relocates the box only; the per-frame
  BuildZoomFrame/ZoomFrameBuilder path is gone.
- SceneCompositor: skips the active ZoomSceneElement in every layer loop and
  BlitZooms it last via the new pure ZoomMagnifier (2x bilinear, temp snapshot
  via ArrayPool, round clip).
- Preview: ZoomBox hoisted above CanvasGrid as a sibling; its brush samples the
  whole composed preview at the fixed capture (2x).
- Tests: ZoomMagnifierTests (box-over-blue shows green capture at 2x incl. an
  extra red "layer" — the loupe/backdrop-only regressions both fail it) +
  round-clip; ZoomBoxInteractionTests rewritten (anchored capture, move-doesn't-
  change-capture, 4:3/9:16/square). 404 pass, 2 known environment flakes.

Controls/PreviewPane.xaml

@@ -63,8 +63,8 @@
                                     </ContextMenu>
                                 </Grid.ContextMenu>
                                 <!-- Live desktop/game background (bottom layer; the static
-                                     image renders above it). The zoom magnifier samples
-                                     this layer — see UpdateZoomBox (TASK 51). -->
+                                     image renders above it). The zoom box samples the
+                                     whole CanvasGrid, not just this layer (TASK 51). -->
                                 <Grid x:Name="BackdropLayer" IsHitTestVisible="False">
                                     <Image Source="{Binding BackgroundImage}" Stretch="UniformToFill"
                                            Visibility="{Binding BackgroundVisible, Converter={StaticResource BoolToVis}}"/>
@@ -72,19 +72,6 @@
                                     <Rectangle Stroke="#22c55e" StrokeThickness="6"/>
                                 </Grid>
                                 <Canvas x:Name="OverlayCanvas" Width="1920" Height="1080">
-                                    <!-- The zoom box renders below every element layer so the
-                                         preview shows exactly what the output composites (the
-                                         zoom is a real layer at z-index 1, not an overlay). -->
-                                    <Grid x:Name="ZoomBox" IsHitTestVisible="False"
-                                          Canvas.Left="{Binding ZoomState.X}" Canvas.Top="{Binding ZoomState.Y}"
-                                          Width="{Binding ZoomState.Width}" Height="{Binding ZoomState.Height}"
-                                          Visibility="{Binding IsZoomActive, Converter={StaticResource BoolToVis}}">
-                                        <Rectangle x:Name="ZoomRectOutline" Stroke="#d8a72b" StrokeThickness="3" Fill="Transparent"/>
-                                        <Ellipse x:Name="ZoomCircleOutline" Stroke="#d8a72b" StrokeThickness="3" Fill="Transparent" Visibility="Collapsed"/>
-                                        <Grid.Background>
-                                            <VisualBrush x:Name="ZoomVisualBrush" ViewboxUnits="Absolute" Stretch="Fill"/>
-                                        </Grid.Background>
-                                    </Grid>
                                     <ItemsControl ItemsSource="{Binding StagedScene.Elements}"
                                                   Width="1920" Height="1080">
                                         <ItemsControl.ItemsPanel>
@@ -422,6 +409,23 @@
                                      </Grid>
                                  </Canvas>
                             </Grid>
+
+                            <!-- The zoom box: layer n+1 — a SIBLING of CanvasGrid (not a
+                                 descendant), so its VisualBrush can sample the whole
+                                 composed preview (CanvasGrid = backdrop + every element
+                                 layer) without a circular reference, and it renders ABOVE
+                                 everything. It shows a fixed capture window (box/2 at the
+                                 click); dragging the box relocates it, never the capture. -->
+                            <Grid x:Name="ZoomBox" IsHitTestVisible="False"
+                                  Canvas.Left="{Binding ZoomState.X}" Canvas.Top="{Binding ZoomState.Y}"
+                                  Width="{Binding ZoomState.Width}" Height="{Binding ZoomState.Height}"
+                                  Visibility="{Binding IsZoomActive, Converter={StaticResource BoolToVis}}">
+                                <Rectangle x:Name="ZoomRectOutline" Stroke="#d8a72b" StrokeThickness="3" Fill="Transparent"/>
+                                <Ellipse x:Name="ZoomCircleOutline" Stroke="#d8a72b" StrokeThickness="3" Fill="Transparent" Visibility="Collapsed"/>
+                                <Grid.Background>
+                                    <VisualBrush x:Name="ZoomVisualBrush" ViewboxUnits="Absolute" Stretch="Fill"/>
+                                </Grid.Background>
+                            </Grid>
                             </Canvas>
                         </Viewbox>
                         <Border HorizontalAlignment="Left" VerticalAlignment="Top" Margin="8,8,0,0"