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

Services/Compositor/AlertTickerRenderer.cs

@@ -34,6 +34,15 @@ public static class AlertTickerRenderer
     /// (0.5s on, 0.5s off — a 1Hz pulse for the whole alert).</summary>
     public const double FlashHalfSeconds = 0.5;
 
+    public static VideoFrame? Render(Ticker ticker, string? text, double elapsedSeconds,
+        double alertSeconds, int readsPerAlert, int width, int height)
+    {
+        if (ticker == null) throw new ArgumentNullException(nameof(ticker));
+        return ticker.IsEnabled
+            ? Render(text, elapsedSeconds, ticker.DisplayMethod, alertSeconds, readsPerAlert, width, height)
+            : null;
+    }
+
     private static readonly object Gate = new();
     private static readonly Dictionary<string, VideoFrame> StripCache = new();
 
@@ -179,4 +188,4 @@ public static class AlertTickerRenderer
         }
         return new VideoFrame(pillW, Height, pixels);
     }
-}
+}