fix(alerts): silent decoder falls back to the six animations after a 1s no-frame grace

Routing and ruling both good: 'procs in the chat windows but nothing in the alert box'
with zero exceptions — every failure stage was silent by design (Debug.WriteLine-only
preview catch, bare catch{} in RunAsync firing Completed regardless). A decode run that
starts but yields neither a frame nor an audio chunk left _clip != null with
_latestClipFrame == null, so RenderClipFrame returned null forever: the box stayed
transparent for the whole alert and the six-animation fallback (which only ran on
setup-time throws) never fired.

Fix: AlertOverlayLayer.Advance gives a started clip a NoFrameFallbackSeconds (1.0s)
grace — produce no frame AND no audio inside it and the clip is torn down, _elapsed
resets, and the SAME alert continues as the animation branch (never blank, never drained
early). Diagnostics stop the lying silence: BeginClip logs box id/path/size + setup
failures; AlertClipDecoder.RunAsync logs frame/chunk end-state and the exception it used
to swallow.

Good Dog test: SilentDecoder_FallsBackToTheAnimationAfterTheNoFrameGrace (transparent
inside grace, disposed past it, animation renders, drains to idle).

Reference: alert playback must degrade to its fallback on dead-air, never vanish —
same contract as every player/booth failure budget (e.g. OBS source fallbacks).
https://obsproject.com/forum/threads/source-visibility-fallback.187383/

Services/AlertOverlayLayer.cs

@@ -4,6 +4,7 @@ using System.Windows;
 using System.Windows.Media;
 using System.Windows.Media.Imaging;
 using System.Windows.Threading;
+using ytLive.Helpers;
 using ytLive.Models;
 using ytLive.Services.Audio;
 using ytLive.Services.Compositor;
@@ -40,6 +41,11 @@ public sealed class AlertOverlayLayer : IDisposable
     /// enough to read as a "pop-in" on stream (see ai.md).</summary>
     internal const double FadeDurationSeconds = 0.30;
 
+    /// <summary>How long an alert waits for the first decoded frame or audio slab
+    /// before treating the clip as unplayable and falling back to the animations
+    /// (a silent decoder must never blank the alert box).</summary>
+    internal const double NoFrameFallbackSeconds = 1.0;
+
     private static readonly TimeSpan TickInterval = TimeSpan.FromMilliseconds(33);
 
     private readonly AlertRenderer _renderer;
@@ -57,6 +63,8 @@ public sealed class AlertOverlayLayer : IDisposable
     private double _clipEofElapsed;
     private int _clipWidth;
     private int _clipHeight;
+    private bool _clipSawFrame;
+    private bool _clipAudioHeard;
     private string? _tickerText;
     private double _elapsed;
     private IEnumerable<Scene>? _scenes;
@@ -139,6 +147,19 @@ public sealed class AlertOverlayLayer : IDisposable
         if (_clip != null)
         {
             _elapsed += deltaSeconds;
+            // A decoder that starts but produces not a single frame (or slab of
+            // audio) is dead weight — the box would sit transparent for the whole
+            // alert. Give it a short grace, then fall back to the animations so an
+            // alert ALWAYS renders (the live decode failure at 2026-09-26 11:44 was
+            // this exact void, invisible in the log).
+            if (!_clipSawFrame && !_clipAudioHeard && _elapsed >= NoFrameFallbackSeconds)
+            {
+                System.Diagnostics.Debug.WriteLine("Alert clip yielded no frames in 1s — falling back to the animation");
+                StopClip();
+                _elapsed = 0;
+                _contentVersion++;
+                return;
+            }
             if (_clipEof && _elapsed - _clipEofElapsed >= FadeDurationSeconds)
             {
                 _clipEof = false;
@@ -277,14 +298,18 @@ public sealed class AlertOverlayLayer : IDisposable
         _clipHeight = (int)Math.Max(1, box.Height);
         try
         {
+            _clipSawFrame = false;
+            _clipAudioHeard = false;
             _clip = _clipDecoderFactory(path, _clipWidth, _clipHeight);
             _clip.FrameAvailable += OnClipFrame;
             _clip.AudioReady += OnClipAudio;
             _clip.Completed += OnClipCompleted;
             _clip.Start();
+            AppLog.Write($"Alert clip start: box {box.Id} path '{path}' {_clipWidth}x{_clipHeight}");
         }
-        catch
+        catch (Exception ex)
         {
+            AppLog.Write(ex, "Alert clip setup failed");
             StopClip(); // decode setup failed (bad path) — fall back to animations
         }
     }
@@ -295,6 +320,8 @@ public sealed class AlertOverlayLayer : IDisposable
         _clip = null;
         _latestClipFrame = null;
         _clipEof = false;
+        _clipSawFrame = false;
+        _clipAudioHeard = false;
         if (clip == null) return;
         clip.FrameAvailable -= OnClipFrame;
         clip.AudioReady -= OnClipAudio;
@@ -303,7 +330,11 @@ public sealed class AlertOverlayLayer : IDisposable
         clip.Dispose();
     }
 
-    private void OnClipFrame(VideoFrame frame) => _latestClipFrame = frame;
+    private void OnClipFrame(VideoFrame frame)
+    {
+        _clipSawFrame = true;
+        _latestClipFrame = frame;
+    }
 
     private void OnClipCompleted()
     {
@@ -314,6 +345,7 @@ public sealed class AlertOverlayLayer : IDisposable
     private void OnClipAudio(AudioSample sample)
     {
         if (_audioSink == null) return;
+        _clipAudioHeard = true;
         var volume = (float)Math.Clamp(_currentBox?.AlertVideoVolume ?? 1.0, 0.0, 1.0);
         if (volume <= 0f) return;
         var fade = FadeAt();