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.

ytLive.Tests/ZoomFrameBuilderTests.cs

@@ -0,0 +1,83 @@
+using System;
+using Xunit;
+using ytLive.Services;
+using ytLive.ViewModels;
+
+namespace ytLive.Tests;
+
+/// <summary>
+/// Proves the zoom layer's pixels are the GENUINE 2x magnification of the
+/// backdrop: the source window is the box/2 interior centred on the box centre
+/// (the same region as the preview's VisualBrush), with the static art composited
+/// over the live capture exactly as the SceneCompositor draws the backdrop. This
+/// is the "don't ship a magnifier you only think exists" proof — a 1x box, a
+/// mis-positioned window, or a wrong backdrop layer all fail hard below.
+/// </summary>
+public sealed class ZoomFrameBuilderTests
+{
+    private static VideoFrame BandedBackdrop(int bandW, int bandCount)
+    {
+        var w = bandW * bandCount;
+        const int h = 1080;
+        var buf = new byte[w * h * 4];
+        var bandColors = new (byte B, byte G, byte R)[]
+        {
+            (10, 20, 30),  // band 0
+            (0, 0, 255),   // band 1 red
+            (0, 255, 0),   // band 2 green — the box window lands here
+            (255, 0, 0),   // band 3 blue
+        };
+        for (var y = 0; y < h; y++)
+        {
+            for (var x = 0; x < w; x++)
+            {
+                var (b, g, r) = bandColors[Math.Min(bandCount - 1, x / bandW)];
+                var i = (y * w + x) * 4;
+                buf[i] = b;
+                buf[i + 1] = g;
+                buf[i + 2] = r;
+                buf[i + 3] = 255;
+            }
+        }
+        return new VideoFrame(w, h, buf);
+    }
+
+    private static VideoFrame SolidFrame(int w, int h, byte b, byte g, byte r)
+    {
+        var buf = new byte[w * h * 4];
+        for (var i = 0; i < w * h; i++)
+        {
+            buf[i * 4] = b;
+            buf[i * 4 + 1] = g;
+            buf[i * 4 + 2] = r;
+            buf[i * 4 + 3] = 255;
+        }
+        return new VideoFrame(w, h, buf);
+    }
+
+    [Fact]
+    public void Build_Produces_2x_Centered_ArtOverLive_Magnification()
+    {
+        var art = BandedBackdrop(480, 4); // [0,480) 1/4 dim, [480,960) red, [960,1440) green, [1440,1920) blue
+        var live = SolidFrame(1920, 1080, 200, 60, 60); // magenta-ish — must never show through
+
+        // Box [840,1320)x[100,580). 2x window = [960,1200)x[160,520) = band 2, all green.
+        // A 1x (broken) magnifier samples [840,1320) → red+green+blue mixed.
+        var zoom = ZoomFrameBuilder.Build(live, art, 840, 100, 480, 480, 1920, 1080);
+
+        Assert.NotNull(zoom);
+        Assert.Equal(480, zoom.Width);
+        Assert.Equal(480, zoom.Height);
+        for (var y = 0; y < zoom.Height; y++)
+        {
+            for (var x = 0; x < zoom.Width; x++)
+            {
+                var i = (y * zoom.Width + x) * 4;
+                Assert.Equal(0, zoom.BgraPixels[i]);       // B
+                Assert.Equal(255, zoom.BgraPixels[i + 1]); // G
+                Assert.Equal(0, zoom.BgraPixels[i + 2]);   // R
+                Assert.Equal(255, zoom.BgraPixels[i + 3]); // A
+            }
+        }
+    }
+}