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.

ViewModels/MainViewModel.Streaming.cs

@@ -411,13 +411,7 @@ public partial class MainViewModel : ViewModelBase
         OutputRectWidth = rectW;
         OutputRectHeight = rectH;
 
-        DimRects.Clear();
-        if (rectX > 0) DimRects.Add(new Rect(0, 0, rectX, MasterFrameHeight));
-        if (rectX + rectW < MasterFrameWidth)
-            DimRects.Add(new Rect(rectX + rectW, 0, MasterFrameWidth - rectX - rectW, MasterFrameHeight));
-        if (rectY > 0) DimRects.Add(new Rect(0, 0, MasterFrameWidth, rectY));
-        if (rectY + rectH < MasterFrameHeight)
-            DimRects.Add(new Rect(0, rectY + rectH, MasterFrameWidth, MasterFrameHeight - rectY - rectH));
+        RebuildQualityDimRects();
 
         IsOutputCropped = DimRects.Count > 0;
         ResolutionBadgeText = $"{quality.Label} · {quality.Width}×{quality.Height}";