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;
 

Models/ZoomSceneElement.cs

@@ -0,0 +1,28 @@
+using System.Windows.Media;
+
+namespace ytLive.Models;
+
+/// <summary>
+/// The transient zoom layer: a real scene element parked at z-index 1 (above the
+/// background, below every other layer) whose pixels are the 2x-magnified backdrop
+/// centred on the zoom box. The app paints it into the stream/recording exactly like
+/// any other layer. While inactive it is a 0x0 invisible placeholder that is never
+/// persisted (LayoutStore hand-rolls only Source/WebcamSceneConfig rows) — the VM
+/// re-inserts it on load and on scene creation, mirroring how backgrounds heal.
+/// </summary>
+public sealed class ZoomSceneElement : SceneElement
+{
+    /// <summary>True only while a zoom box is live; flips Kind so the layer leaves
+    /// the baked static base and composites per-frame.</summary>
+    public bool IsZoomActive { get; set; }
+
+    public override ElementKind Kind => IsZoomActive ? ElementKind.Dynamic : ElementKind.Static;
+
+    public override bool IsZoomLayer => true;
+
+    public override bool IsDraggable => false;
+
+    /// <summary>Never rendered by the element template — the preview magnifier
+    /// (ZoomBox) and the output resolver own its pixels.</summary>
+    public override ImageSource? DisplaySource => null;
+}

Models/index.md

@@ -8,7 +8,8 @@ Plain data types. No logic beyond what a property can carry. See
 | `Scene.cs` | A scene: `Name`, `IsHidden`, `IsChatScene`, `IsEditing`, **`HasBackdrop`** (Live-only policy flag, default off — only the canonical Live scene has one, see `SceneCatalog`), `Elements` collection (images + webcam config), `WebcamConfig` accessor |
 | `SceneCatalog.cs` | The five canonical scenes (Starting/Live/BRB/Chat/Ending) — the product, by name: `All`, `IsCanonical`, `Is(name, canonical)` (case-insensitive trim), `IsChat`, `HasBackdrop(name)` (true only for Live). Drives the empty-DB seed, the "+" missing-scenes menu, and `MainViewModel.EnforceBackdropPolicy` |
 | `Source.cs` | A source: `SourceType` enum (DisplayCapture/WindowCapture/Background/Image/TextOverlay/ChatBox/Countdown/WebSource/MediaSource/**AlertBox (TASK 43 — the celebration zone, one per layout)** — **Webcam removed**), `X`/`Y`/`Width`/`Height`, `Opacity`, `IsEnabled`, `ImageSource` (static) / `VideoImageSource` (live frames), `DisplaySource` (`IsLiveCapture` → video, else static), `ClipShape` enum (Traditional/Round), `IsMirrored` + `MirrorScale`, `MirrorButtonText`/`ShapeButtonText`, asset identity, `IsImageSource` (XAML binds this, never `Type`), **backdrop surface**: `IsBackdrop` (permanent bottom layer, never removable), `CaptureKey` (persisted `monitor:<n>`/`window:<hwnd>`/`picker:<name>`), `IsLiveCapture` (`Type` is Display/WindowCapture) |
-| `SceneElement.cs` | Base for anything placeable in a scene: shared layout/clip/mirror/border surface, `virtual IsWebcam`/`virtual IsImageSource`; `ToggleClipShape` + persisted `RectWidth`/`RectHeight` (pre-Round rect so round→rect restores after reload) |
+| `SceneElement.cs` | Base for anything placeable in a scene: shared layout/clip/mirror/border surface, `virtual IsWebcam`/`virtual IsImageSource`/`virtual IsZoomLayer` (TASK 51); `ToggleClipShape` + persisted `RectWidth`/`RectHeight` (pre-Round rect so round→rect restores after reload) |
+| `ZoomSceneElement.cs` | **Transient zoom layer (TASK 51):** a real composited element parked at z-index 1 (above the background, below every other layer) whose pixels are the 2x-magnified backdrop? — the frame pump builds them via `ZoomFrameBuilder` and composites like any layer. `IsZoomActive` flips `Kind` Static→Dynamic (leaves the baked base while active); never persisted (LayoutStore save skips non-Source/non-WebcamSceneConfig rows), never shown in the layers list or the element template (`IsZoomLayer` gates both) |
 | `Webcam.cs` | Singleton webcam identity: `Id`, `DeviceId`, `Name` (one row app-wide) |
 | `WebcamSceneConfig.cs` | Per-scene webcam placement (subclass of `SceneElement`): geometry + `IsVisible` + border (`BorderColor`/`BorderOpacity`/`BorderWidth`/`BorderAnimation`) + `VideoImageSource`; `WebcamId` links to `Webcam` |
 | `QualityOption.cs` | Resolution tier record `(Display, Fps, Bitrate, Width, Height)`; `Label` formats as `1080p60 (8Mbps)` — drives the bottom-bar dropdown and the output-rect math |