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.

Controls/LeftPanel.xaml

@@ -762,8 +762,11 @@
                                 <TextBlock Text="Sound plays through the desktop audio controls, like game audio."
                                            Style="{StaticResource PropertyHint}"/>
 
-                                <!-- Green/Blue Screen (TASK 64): the app offers, the creator
-                                     accepts. No engineering words — Amount/Softness/Fringe. -->
+</StackPanel>
+
+                            <!-- Green/Blue Screen (TASK 64): a video layer OR the alert box's
+                                 own clip. The app offers, the creator accepts — never silent. -->
+                            <StackPanel Visibility="{Binding SelectedElement.ShowsChromaKey, Converter={StaticResource BoolToVis}}" Margin="0,0,0,8">
                                 <Border Style="{StaticResource PropertyDivider}"/>
                                 <TextBlock Text="Green/Blue Screen" Style="{StaticResource PropertyHeader}"/>
 
@@ -787,6 +790,9 @@
                                 <CheckBox Style="{StaticResource PropertyPillCheck}" Margin="10,0,0,4"
                                           Content="Remove the background" IsChecked="{Binding SelectedElement.ChromaKeyEnabled, Mode=TwoWay}"
                                           ToolTip="Key out the green/blue backdrop so the layers beneath show through"/>
+                                <TextBlock Text="Removed automatically — green screen detected."
+                                           Style="{StaticResource PropertyHint}"
+                                           Visibility="{Binding SelectedElement.HasAutoChromaKey, Converter={StaticResource BoolToVis}}"/>
                                 <DockPanel Margin="10,0,0,4">
                                     <TextBlock DockPanel.Dock="Left" Text="Amount" Style="{StaticResource PropertyFieldLabel}" Width="52"/>
                                     <Slider Minimum="0" Maximum="0.5" Width="110"