feat(alerts): add scene-local recent ticker

Model the AlertBox and standalone ticker with shared settings; scene tickers render the in-memory active and queued events without saving history. Keep AlertBox content active-event-only.

Ref: https://obsproject.com/kb/obs-studio-overview

ytLive.Tests/AlertTickerPreviewDisplayTests.cs

@@ -436,6 +436,68 @@ public sealed class AlertTickerPreviewDisplayTests
         });
     }
 
+    [Fact]
+    public void SceneTickersKeepIndependentSettings_AndShowQueueWhileAlertBoxShowsActiveEvent()
+    {
+        _app.Run(() =>
+        {
+            const int width = 680;
+            const int height = 48;
+            var alertBox = new Source
+            {
+                Name = "Stream Alerts",
+                Type = SourceType.AlertBox,
+                Width = width,
+                Height = height,
+            };
+            var tickerA = new Source
+            {
+                Name = "Ticker A",
+                Type = SourceType.Ticker,
+                Width = width,
+                Height = height,
+            };
+            var tickerB = new Source
+            {
+                Name = "Ticker B",
+                Type = SourceType.Ticker,
+                Width = width,
+                Height = height,
+            };
+            tickerA.Ticker.DisplayMethod = AlertDisplayMethod.Solid;
+            tickerB.Ticker.DisplayMethod = AlertDisplayMethod.Flash;
+            alertBox.Ticker.DisplayMethod = AlertDisplayMethod.Solid;
+            var firstScene = new Scene { Name = "Starting", Elements = { alertBox, tickerA } };
+            var secondScene = new Scene { Name = "BRB", Elements = { tickerB } };
+            using var layer = new AlertOverlayLayer(new AlertRenderer(), tickerPreviewSink: _ => { });
+            layer.UpdateConfiguredBoxes(new[] { firstScene, secondScene });
+            layer.Enqueue(new ChatMessage
+            {
+                AuthorName = "Active",
+                Kind = ChatEventKind.SuperChat,
+                SuperChatDisplayString = "$10.00",
+            });
+            layer.Enqueue(new ChatMessage
+            {
+                AuthorName = "Queued",
+                Kind = ChatEventKind.NewMember,
+            });
+            layer.Advance(0.7);
+
+            var activeAlert = layer.AlertBoxTickerFrame(alertBox);
+            var recentFeed = layer.RenderSceneTicker(tickerA);
+
+            Assert.NotSame(tickerA.Ticker, tickerB.Ticker);
+            Assert.Equal(AlertDisplayMethod.Solid, tickerA.Ticker.DisplayMethod);
+            Assert.Equal(AlertDisplayMethod.Flash, tickerB.Ticker.DisplayMethod);
+            Assert.NotNull(activeAlert);
+            Assert.NotNull(recentFeed);
+            Assert.Null(layer.RenderSceneTicker(tickerB));
+            Assert.False(activeAlert!.BgraPixels.SequenceEqual(recentFeed!.BgraPixels),
+                "the standalone feed must include the queued event while Stream Alerts shows only the active event");
+        });
+    }
+
     /// <summary>A short alert box must clip the strip's rows, not overrun the target
     /// buffer. The strip rasterises at its natural 48px, so a 20px-tall box used to be
     /// a heap-corrupting write once the strip was composited at the box's size.</summary>