Zoom: fix preview zoom box + shape-aspect anchoring (Unit: zoom box visuals)

Right-click in the live preview now procs a box that is actually visible and
populated with the zoomed region, and each Zoom View Shape sculpts the box per
spec. OBS-style derivative: magnifier box shows the composited region and dims
the surroundings (same pattern as OBS "zoom" / preview region highlights).

Root causes of the invisible box (two WPF traps, recorded in MyMistakes.md):
- Empty <VisualBrush Stretch="None"/> used as an OpacityMask paints nothing,
  so the mask alpha is 0 and the WHOLE box (border included) went transparent.
- ZoomBox lived inside CanvasGrid while its VisualBrush sampled CanvasGrid — a
  circular reference WPF renders as nothing. Hoisted ZoomBox to a sibling of
  CanvasGrid inside a new PreviewCanvas wrapper (same 1920x1080 space).

Shape semantics (MainViewModel.ActivateZoom):
- Circle: square, click point = center.
- VerticalRectangle (phone): tall 9:16, click point = top-left corner.
- HorizontalRectangle (tablet): wide 16:9, click point = top-left corner.
- Boxes stay fully inside 1920x1080 via Math.Clamp.

UI: ZoomRectOutline/ZoomCircleOutline shapes replace the square Border;
circle clips the box to an EllipseGeometry; brush Visual released when inactive.
Zoom View Shape dropdown now uses the standard YtComboBox style.

Test (Good Dog rule, one integration test): ZoomBoxInteractionTests verifies
aspect/anchoring/clamp per shape and the matching outline+clip in the real
MainWindow. Full suite 404 passed, 0 warnings.

Memory: HANDOFF.md zoom state updated; WPF VisualBrush traps appended to
MyMistakes.md.

Controls/OverlayHost.xaml

@@ -1,7 +1,8 @@
 <UserControl x:Class="ytLive.Controls.OverlayHost"
              xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
              xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
-             xmlns:Helpers="clr-namespace:ytLive.Helpers">
+             xmlns:Helpers="clr-namespace:ytLive.Helpers"
+             xmlns:Models="clr-namespace:ytLive.Models">
         <Grid Grid.RowSpan="3"
               Visibility="{Binding IsAnyOverlayOpen, Converter={StaticResource BoolToVis}}">
             <Button Background="#99000000" BorderThickness="0" Cursor="Hand"
@@ -42,12 +43,18 @@
                         <TextBox Style="{StaticResource YtTextBox}" 
                                  Text="{Binding RecordFolderDisplay, Mode=OneWay}"
                                  IsReadOnly="True" Margin="0,0,0,12" Foreground="#a0a0b0"/>
-                        <StackPanel Orientation="Horizontal">
+                        <StackPanel Orientation="Horizontal" Margin="0,0,0,16">
                             <Button Content="Browse…" Command="{Binding ChooseRecordFolderCommand}"
                                     Style="{StaticResource YtButtonSecondary}" Margin="0,0,8,0"/>
                             <Button Content="Use Downloads" Command="{Binding ResetRecordFolderCommand}"
                                     Style="{StaticResource YtButtonSecondary}"/>
                         </StackPanel>
+                        <TextBlock Text="Zoom View Shape" Style="{StaticResource YtLabel}" Margin="0,0,0,4"/>
+                        <ComboBox Style="{StaticResource YtComboBox}" SelectedValue="{Binding ZoomShape}" SelectedValuePath="Tag">
+                            <ComboBoxItem Content="Circle" Tag="{x:Static Models:ZoomShape.Circle}"/>
+                            <ComboBoxItem Content="Phone (Vertical Rectangle)" Tag="{x:Static Models:ZoomShape.VerticalRectangle}"/>
+                            <ComboBoxItem Content="Tablet (Horizontal Rectangle)" Tag="{x:Static Models:ZoomShape.HorizontalRectangle}"/>
+                        </ComboBox>
                     </StackPanel>
 
                     <!-- Report Bug -->