feat(chroma): opt-in auto key, and alert clips key like any other clip

TASK 64 slice F — the two things left after the feature worked by hand.

- App Settings → **Video clips → Auto Green/Blue Screen** (off by default). When on, a
  high-confidence detected screen is keyed as soon as the clip is added; the panel says
  "Removed automatically" so an automatic change is never silent. Medium confidence still
  asks, low says nothing — a wrong automatic key eats the picture.
- **A clip is a clip:** the alert box's own replacement clip keys through the same settings
  block (one alert box per layout, so its settings are the ones the next alert uses), and
  choosing an alert clip runs the same detector. The Green/Blue Screen panel now appears for
  a video layer *or* an alert box with a custom clip (`Source.ShowsChromaKey`).
- Spill ships as the creator-facing **Fringe** switch; nothing further to tune behind it.

Test: `ChromaKeyApplyTests` gains the auto-apply case (no click, still explicit in the panel)
and an alert-clip case (its pipe asks ffmpeg for colorkey+despill with the blue screen type).

Docs: TASK 64 complete in TASKS.md, known limits recorded in the task file, HANDOFF rewritten.
verify.sh: 546 passed, 0 failed, 0 warnings.

Models/Source.cs

@@ -111,6 +111,7 @@ public class Source : SceneElement
                 Raise(nameof(IsTextOverlay));
                 Raise(nameof(IsScoreboard));
                 Raise(nameof(IsTicker));
+                Raise(nameof(ShowsChromaKey));
             }
         }
     }
@@ -379,6 +380,25 @@ public class Source : SceneElement
     /// <summary>True while there is an offer the creator has not accepted.</summary>
     public bool HasChromaSuggestion => !ChromaKeyEnabled && ChromaSuggestion.Screen != ChromaScreen.None;
 
+    private bool _chromaKeyAutoApplied;
+    /// <summary>True when the app applied the key on its own (the "Auto Green/Blue Screen"
+    /// setting). The panel says so, so an automatic change is never silent.</summary>
+    public bool ChromaKeyAutoApplied
+    {
+        get => _chromaKeyAutoApplied;
+        set
+        {
+            if (Set(ref _chromaKeyAutoApplied, value)) Raise(nameof(HasAutoChromaKey));
+        }
+    }
+
+    public bool HasAutoChromaKey => ChromaKeyAutoApplied && ChromaKeyEnabled;
+
+    /// <summary>Which layers get the Green/Blue Screen block: a video layer, or the alert box's
+    /// own clip (TASK 64 — a clip is a clip).</summary>
+    public bool ShowsChromaKey => IsMediaSource
+        || (Type == SourceType.AlertBox && !AlertUseDefaultVideo && !string.IsNullOrWhiteSpace(AlertVideoPath));
+
     public string ChromaSuggestionText => ChromaSuggestion.Screen switch
     {
         ChromaScreen.Green => "Looks like a green screen.",
@@ -406,14 +426,20 @@ public class Source : SceneElement
     public string? AlertVideoPath
     {
         get => _alertVideoPath;
-        set => Set(ref _alertVideoPath, value);
+        set
+        {
+            if (Set(ref _alertVideoPath, value)) Raise(nameof(ShowsChromaKey));
+        }
     }
 
     private bool _alertUseDefaultVideo = true;
     public bool AlertUseDefaultVideo
     {
         get => _alertUseDefaultVideo;
-        set => Set(ref _alertUseDefaultVideo, value);
+        set
+        {
+            if (Set(ref _alertUseDefaultVideo, value)) Raise(nameof(ShowsChromaKey));
+        }
     }
 
     public bool AlertShowTicker