Zoom: dims never shade the box — move the dim overlay above CanvasGrid (Unit: zoom shading fix)

Creator's final zoom correction: as the box is dragged, its capture region slides
under the dim rectangles and the box picks up the 55% black "div shading" inside
itself, because the VisualBrush samples the whole CanvasGrid and DimRects lived
inside CanvasGrid. The captured area must stay clear regardless of box position.

- PreviewPane.xaml: the DimRects ItemsControl is now a SIBLING of CanvasGrid
  (above the composed content, below the ZoomBox), never a descendant — the zoom
  source is shading-free by construction.
- ZoomBoxInteractionTests: structural invariant — DimRectsLayer is a direct
  child of PreviewCanvas (not inside CanvasGrid) and renders below the ZoomBox.
- Side effect (correctness, not a bug): quality dims now also sedate the global
  overlays outside the crop, matching "chrome outside the crop is dimmed".

ai.md

@@ -737,6 +737,11 @@ parallel software compositor** over the `VideoFrame` (BGRA8) seam, and the XAML
 renderers must agree: geometry, `UniformToFill` cover-crop, round clip, mirror, border, z-order. Preview
 stays XAML (editing view); the compositor is the output view.
 
+- **The `DimRects` shading is itself preview chrome, so it renders as a SIBLING of `CanvasGrid`
+  (above it, below the `ZoomBox`), never a descendant** (2026-10-08, creator ruling): the zoom box's
+  `VisualBrush` samples all of `CanvasGrid`, and if the dims were inside it the box would show the
+  dim shading — especially after the box is dragged away and the fixed capture slides under the dims.
+
 - **Render the active tier's output rect directly** (`CompositorOptions {SourceRectX/Y/W/H,
   OutputWidth, OutputHeight}`, fed from `MainViewModel.OutputRect*`): 16:9 = full 1920×1080 1:1; the
   vertical 9:16 tier = composite the centered 607×1080 crop then bilinear-upscale to 1080×1920.