feat(alerts): TASK 47 — alert box plays a video (built-in/custom clip) + read-time fade + message ticker

TASK 43's alert box grows a real video celebration. Per-alert IAlertClipDecoder
(ffmpeg bgra + f32le pipes, real-time paced, disposed at drain) plays the shipped
Assets/alert-default.mp4 (stamped into the Asset table at startup) unless the
creator picks their own file — path reference only, never stored in the DB; the
six AlertRenderer animations stay the fallback. ~0.3s fade rides the alpha
envelope on straight-source copies (EOF freeze-frames then fades out); audio
forwards to a new AudioMixer alert ring (8s, 48k stereo) drained at unity — no
duck, creator ruling — scaled by volume × fade. An auto-composed marquee ticker
('Funder — Super Chat · $10.00', 140px/s) scrolls top-of-frame via a
FramePump._alertTicker seam through Render/CompositeLayers, mixed into the cache
signature (dynamic overlay, never baked). New Stream Alerts section in LeftPanel.

Derivative-work references (how OBS/Streamlabs alert boxes do per-alert video):
- https://support.streamlabs.com/hc/en-us/articles/217741147-Setting-Up-Your-Streamlabs-Alerts (custom image/video per alert type + variations)
- https://obsproject.com/kb/stream-tutorial-2-alerts (alert overlay as an on-screen zone)
- https://streamlabs.com/content-hub/widgets/alert-box (per-event alert playback)

Good Dog: AlertLayerVideoTests drives a fake IAlertClipDecoder through the whole
lifecycle in one pass (custom path wins, decoder spawns/disposes, fade envelope
0→127→255, audio volume×fade, ticker scrolls, EOF fade-drain to idle). It caught
the clip branch of Advance not clearing _current before AdvanceToNext — the layer
stayed IsPlaying after drain (MyMistakes post-mortem).

Full vstest 319/319; clean build 0 warnings; scope check green.

Services/LayoutStore.Settings.cs

@@ -304,4 +304,26 @@ public partial class LayoutStore : IDisposable
         insert.Parameters.AddWithValue("$value", value);
         insert.ExecuteNonQuery();
     }
+
+    internal const string AlertDefaultVideoKey = "AlertDefaultVideoAssetId";
+
+    /// <summary>Stamps the shipped stream-alert placeholder clip (TASK 47): like any
+    /// source image it lives as a BLOB in the internal Asset table, tracked by this
+    /// Setting so the Save pruner keeps it alive. The creator's OWN replacement
+    /// videos are never stored — they are plain path references (the 2026-09-26
+    /// decision: the file the user picks stays at its real location on disk).
+    /// <c>UpsertAsset</c> is hash-keyed, so re-stamping the same bytes returns the
+    /// same id.</summary>
+    public string EnsureAlertDefaultVideo(byte[] data)
+    {
+        var saved = GetSetting(AlertDefaultVideoKey);
+        if (!string.IsNullOrWhiteSpace(saved) && GetAssetBytes(saved) != null)
+            return saved;
+        var id = UpsertAsset(data, 0, 0);
+        UpsertSetting(AlertDefaultVideoKey, id);
+        return id;
+    }
+
+    /// <summary>The stored default clip's Asset id, or null when never stamped.</summary>
+    public string? GetAlertDefaultVideoId() => GetSetting(AlertDefaultVideoKey);
 }