alert ticker: draw it inside the Stream Alerts box, not as a top-edge bar

Creator 2026-09-26: "the ticker should appear over the stream alerts video, not over
the entire preview window". It was a GLOBAL 1920x48 bar blitted last at a hardcoded
(0, 0) -- it covered the whole preview width, not the alert video.

The strip is now rendered at the alert box's own size and the box's origin travels on
the frame in a new VideoFrame.Placement ((int X, int Y)?). OriginX/OriginY default to
0 when Placement is null, so every full-canvas overlay (the branding flash) is
unaffected. Every ticker blit site now reads tickerFrame.OriginX/OriginY instead of a
literal 0, 0 -- SceneCompositor x2 and FramePump's static-bake Overlay path -- and
PreviewPane.xaml's AlertTickerElement binds the same rect (AlertTickerLeft/Top/
Width/Height), so the preview cannot drift from the recording the creator is judging.

Why the position rides on the frame rather than in a full-canvas frame: a 1920x1080
overlay is 8.3MB of large-object-heap garbage per tick, ~2.5GB churned over one 10s
alert at 30fps. A box-sized strip is ~370KB. The branding flash does render
full-canvas but RECYCLES one 8MB buffer and bumps Epoch, so it never allocates per
frame; the ticker allocates per call, so it has to stay small.

Also fixed: CopyStrip now clips ROWS to the target height. The pill rasterises at its
natural 48px, so an alert box shorter than that would have written past the end of the
target buffer once the strip became box-sized.

No alert box in the scene now means no ticker at all -- there is no global position
left for it, and this is the guard against the old bar quietly coming back.

Tests (3 new facts, 27 in the file):
  ComposedOutput_PutsTheTickerInsideTheAlertBox_NotAtTheTopEdge -- renders through
    the real SceneCompositor and asserts the pixels land at (620, 430) and NOT at the
    top-left corner. The creator is judging compositing from local recordings, so the
    OUTPUT side is the side that needed proving.
  ASceneWithNoAlertBox_PublishesNoTicker
  ABoxShorterThanTheStrip_ClipsRowsInsteadOfOverrunning
Updated AlertLayer_PublishesARealTickerFrameToThePreviewSink, which asserted the old
1920 width from a box with no geometry (defaulted to 1px); it now uses the product's
own default box (680x200 at 620,430, MainViewModel.Sources.cs:54-57) and pins the
frame size and origin.

Full suite 367/367.

Services/AlertOverlayLayer.cs

@@ -237,17 +237,38 @@ public sealed class AlertOverlayLayer : IDisposable
         return RenderCore(alertBox, width, height, key, _contentVersion);
     }
 
-    /// <summary>The global ticker strip for the stream output (top edge, full
-    /// master width) and the preview pane: a marquee, a pulse or a static pill
-    /// naming the event while an alert plays, else null. Pure byte-math per call so
-    /// the frame pump can read it on any thread (the text strip is rasterized once
-    /// on the UI thread and cached).</summary>
+    /// <summary>The ticker strip as the alert box shows it: a marquee, a pulse or a
+    /// static pill naming the event while an alert plays, else null. Pure byte-math
+    /// per call so the frame pump can read it on any thread (the text strip is
+    /// rasterized once on the UI thread and cached).
+    /// <para><b>Creator ruling 2026-09-26:</b> the strip is composited INSIDE the
+    /// Stream Alerts box, not as a full-width bar at the top edge of the frame — "the
+    /// ticker should appear over the stream alerts video, not over the entire preview
+    /// window". So the frame is rendered at the box's own size and carries the box's
+    /// origin on <see cref="VideoFrame.Placement"/>; every blit site reads
+    /// <see cref="VideoFrame.OriginX"/>/<see cref="VideoFrame.OriginY"/> instead of
+    /// a hardcoded 0. <b>No alert box in the scene means no ticker at all</b> — there
+    /// is no longer a global position for it to live in.</para></summary>
     public VideoFrame? AlertTickerFrame
     {
         get
         {
             if (_current == null || string.IsNullOrWhiteSpace(_tickerText)) return null;
-            return AlertTickerRenderer.Render(_tickerText!, _elapsed, _tickerMethod, AlertSeconds());
+            var box = _currentBox;
+            if (box == null) return null;
+
+            var w = (int)Math.Max(1, Math.Round(box.Width));
+            var h = (int)Math.Max(1, Math.Min(
+                Math.Round(box.Height), AlertTickerRenderer.Height));
+            var frame = AlertTickerRenderer.Render(
+                _tickerText!, _elapsed, _tickerMethod, AlertSeconds(),
+                TickerReadsPerAlert, w, h);
+            return frame == null
+                ? null
+                : new VideoFrame(frame.Width, frame.Height, frame.BgraPixels)
+                {
+                    Placement = ((int)Math.Round(box.X), (int)Math.Round(box.Y)),
+                };
         }
     }