feat(webcam): resource lifecycle startup slice — poll-on-start, single-cam lock, persistent Layers alert

The creator couldn't add a webcam to Live (grayed app-wide) and nothing in the
app explained why. Ground truth from the live DB: one Webcam identity AND one
WebcamSceneConfig in the Chat scene — so the gray was the single-identity rule
working, but the reason was unobservable. This slice makes the webcam a
resource the app validates and locks, mirroring how OBS reserves its devices.

At startup we enumerate the OS once (ValidateWebcamResourceStartupAsync, fired
after LoadLayout, stored as WebcamStartupValidationTask for tests to await):
- 0 webcams -> app runs on, layer inactive, no alarm
- exactly 1 -> attempt CameraManager.AcquireAsync as an app-wide lock; on
  failure show a persistent red alert at the bottom of the Layers panel
  (WebcamLockAlert + Retry) that re-polls every 5s and clears itself the
  moment the camera locks, or on any first real frame
- >=2 -> deliberately no auto-lock; camera selection belongs to the App
  Settings dialog (gear) — next slice

CameraManager.IsRunning(deviceId) tells the pass a session already exists
(started OR still starting) so loaded identity configs count as the lock and
the pass never double-acquires. Test seams mirror LayoutPathOverride:
CameraEnumeratorOverride / CameraFrameSourceFactoryOverride so the startup
probe never touches real hardware under test.

Good Dog test: WebcamStartupResourceTests x3 — single-cam locked + app runs on,
zero-cams no-alarm/no-lock, lock-fails -> red alert -> Retry -> clears. Full
suite 304/304, build 0 warnings, scope-check passed.

Docs in-commit: TASKS Open items + ai.md Webcam section + HANDOFF rewrite.
Also corrects the record: 'NVIDIA Broadcast opens the webcam exclusively' was a
suspect-list claim (CameraConflictProbe reads process names only, no handles)
— not restated as fact.

Controls/LeftPanel.xaml

@@ -17,6 +17,7 @@
                             <RowDefinition Height="Auto"/>
                             <RowDefinition Height="Auto"/>
                             <RowDefinition Height="Auto"/>
+                            <RowDefinition Height="Auto"/>
                             <RowDefinition Height="*"/>
                         </Grid.RowDefinitions>
 
@@ -167,15 +168,30 @@
                                HorizontalAlignment="Center" VerticalAlignment="Center"
                                Visibility="{Binding ShowSourcesEmptyHint, Converter={StaticResource BoolToVis}}"/>
 
+                    <!-- Resource alert: pinned to the bottom of the Layers panel.
+                         Red when a required resource (the one-and-only webcam)
+                         could not be locked; persists and re-polls until resolved. -->
+                    <Border Grid.Row="3" Margin="0,8,0,0" Padding="8,6" CornerRadius="4"
+                            Background="#3a1216" BorderBrush="#e94560" BorderThickness="1"
+                            Visibility="{Binding WebcamLockAlert, Converter={StaticResource NotNullToVis}}">
+                        <StackPanel>
+                            <TextBlock Text="{Binding WebcamLockAlert}" Foreground="#ffb4b4" FontSize="11"
+                                       TextWrapping="Wrap"/>
+                            <Button Content="Retry" Command="{Binding RetryWebcamLockCommand}"
+                                    Style="{StaticResource YtButtonSecondary}" Padding="10,2"
+                                    HorizontalAlignment="Right" Margin="0,6,0,0"/>
+                        </StackPanel>
+                    </Border>
+
                     <!-- ELEMENTS panel: per-element config beneath Sources.
                          Collapses entirely when no element is selected. -->
-                    <Rectangle Grid.Row="3" Height="1" Fill="#2c3a5c" Margin="0,10,0,2"
+                    <Rectangle Grid.Row="4" Height="1" Fill="#2c3a5c" Margin="0,10,0,2"
                                Visibility="{Binding SelectedElement, Converter={StaticResource NotNullToVis}}"/>
-                    <StackPanel Grid.Row="4" Orientation="Horizontal" Margin="0,8,0,0"
+                    <StackPanel Grid.Row="5" Orientation="Horizontal" Margin="0,8,0,0"
                                 Visibility="{Binding SelectedElement, Converter={StaticResource NotNullToVis}}">
                         <TextBlock Text="PROPERTIES" Style="{StaticResource SectionHeader}" VerticalAlignment="Center"/>
                     </StackPanel>
-                    <ScrollViewer x:Name="ElementsScrollViewer" Grid.Row="5" VerticalScrollBarVisibility="Auto" Background="Transparent"
+                    <ScrollViewer x:Name="ElementsScrollViewer" Grid.Row="6" VerticalScrollBarVisibility="Auto" Background="Transparent"
                                   Visibility="{Binding SelectedElement, Converter={StaticResource NotNullToVis}}">
                         <StackPanel x:Name="ElementsPanel" Margin="0,4,0,0">
                             <!-- Opacity + mirror/shape (draggable elements: image, webcam) -->