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.

ViewModels/MainViewModel.cs

@@ -139,6 +139,8 @@ public partial class MainViewModel : ViewModelBase
     public ICommand RemoveSourceCommand { get; }
     public ICommand EditElementCommand { get; }
     public ICommand ToggleElementVisibilityCommand { get; }
+    public ICommand BrowseAlertVideoCommand { get; }
+    public ICommand ResetAlertVideoCommand { get; }
     public ICommand ActivateLicenseCommand { get; }
     public ICommand OpenLicenseEntryCommand { get; }
     public ICommand OpenHotkeyConfigCommand { get; }
@@ -187,6 +189,8 @@ public partial class MainViewModel : ViewModelBase
         RemoveSourceCommand = new RelayCommand(element => RemoveElement(element as SceneElement));
         EditElementCommand = new RelayCommand(element => BeginEditElement(element as SceneElement));
         ToggleElementVisibilityCommand = new RelayCommand(element => ToggleElementVisibility(element as SceneElement));
+        BrowseAlertVideoCommand = new RelayCommand(element => BrowseAlertVideo(element as Source), element => element is Source { Type: SourceType.AlertBox });
+        ResetAlertVideoCommand = new RelayCommand(element => ResetAlertVideo(element as Source), element => element is Source { Type: SourceType.AlertBox });
         ChangeWebcamCommand = new RelayCommand(_ => _ = ChangeWebcamAsync(), _ => CanChangeWebcam);
         ShowWebcamCommand = new RelayCommand(_ => ShowWebcamInStagedScene(), _ => CanShowWebcamInStagedScene);
         OpenSocialDialogCommand = new RelayCommand(_ => OpenSocialDialog());
@@ -308,7 +312,11 @@ public partial class MainViewModel : ViewModelBase
         _mediaManager.MediaFailed += OnMediaFailed;
 
         _chatLayer = new ChatOverlayLayer(new ChatBoxRenderer());
-        _alertLayer = new AlertOverlayLayer(new AlertRenderer());
+        _alertLayer = new AlertOverlayLayer(
+            new AlertRenderer(),
+            clipDecoderFactory: AlertClipDecoderFor,
+            clipPathResolver: ResolveAlertClipPath,
+            audioSink: _audioMixer.EnqueueAlertAudio);
 
         _rtmpUrlProvider = () => _reusableStreamUrl; // TASK 5: the reusable stream's ingest URL
         _framePump = new FramePump(
@@ -319,12 +327,14 @@ public partial class MainViewModel : ViewModelBase
             encoderFactory: () => new FfmpegEncoder(new FfmpegLocator()),
             log: message => AppLog.Write(message),
             socialBar: () => (_socialBarFrame, _socials?.BarPosition ?? SocialBarPosition.Bottom),
+            alertTicker: () => _alertLayer.AlertTickerFrame,
             transition: _transition,
             sceneGraph: _sceneGraph);
         _framePump.Failed += OnFramePumpFailed;
         _framePump.HealthUpdated += OnFramePumpHealthUpdated;
 
         LoadLayout();
+        StampDefaultAlertVideo();
         // Resource validation runs as soon as the layout is up: enumerate the
         // OS once, and if exactly one webcam exists, lock it for the session
         // (the saved identity's scene configs may already hold it — refcounted).