Zoom: compose the box as a real 2x layer above the background (Unit: zoom layer + magnification proof)

Right-click now creates a transient ZoomSceneElement at z-index 1 whose pixels are a
genuine 2x magnified sample of the backdrop (live capture under art), composited into
stream/recording like any layer while the output crop stays the tier. All shapes are
centered on the click and clamped to frame; healed on load/scene-create, never persisted,
hidden from the layers list + element template (IsZoomLayer). BuildCompositorOptions is
now internal = always tier crop (test seam). ZoomFrameBuilder is a pure static sampler
(box/2 window centered on the box center, single-slot cache) proven by a banded-backdrop
test; LayerReorderPersistenceTests drag resolves rows by item since the collapsed zoom
row shifts container indices. Also books TASK 51 (webcam background Remove + Blur) and
syncs memory docs (HANDOFF, MyMistakes, indexes). Full suite: 405 passed, 0 warnings.

Models/SceneElement.cs

@@ -60,6 +60,10 @@ public abstract class SceneElement : INotifyPropertyChanged
     /// Dynamic layer once (see <c>SceneGraph.GetSplitPoint</c>).</summary>
     public virtual ElementKind Kind => ElementKind.Static;
 
+    /// <summary>True only for the transient zoom layer. Gates zoom-only UI:
+    /// hidden from the layers list, never selectable/draggable in the preview.</summary>
+    public virtual bool IsZoomLayer => false;
+
     /// <summary>True only for singleton resources (webcam). Gates webcam-only UI.</summary>
     public virtual bool IsWebcam => false;