feat(webcam): app-default gate slice — per-scene offer, Add places default directly, identity survives removal

Chat's camera no longer greys Web Cam in Live (per-scene max, not app-wide);
TASK 26 superseded by creator directive (app-level resource model). Add Webcam
now places the existing app default without the picker; the picker runs only
for the initial selection. Removing the last placement keeps the identity
(_webcam never nulled) so Add stays offered. Startup pass adopts a solo camera
as the app default, so a clean layout offers the layer in Live/Chat at once.
Dynamic WebcamAddToolTip names the why (incl. the 'graduated to OBS' line).

Good Dog: WebcamMenuGateTests rewritten (real app + temp DB + camera seams) —
identity in Chat does not gray Live; Add in Live places same wc-1 no picker;
scene-with-placement stays gray; identity survives both removals (DB row 1).
WebcamStartupResourceTests single-lock fact asserts CanAddWebcam after adoption.
Docs same commit (ai.md supersession, TASKS.md, HANDOFF.md). 304/304, 0 warnings.

Controls/LeftPanel.xaml

@@ -30,7 +30,7 @@
                                 <ContextMenu DataContext="{Binding PlacementTarget.DataContext, RelativeSource={RelativeSource Self}}">
                                     <MenuItem Header="Webcam" Command="{Binding AddWebcamCommand}"
                                               IsEnabled="{Binding CanAddWebcam}"
-                                              ToolTip="One webcam at a time — it's already in your stream"/>
+                                              ToolTip="{Binding WebcamAddToolTip}"/>
                                     <MenuItem Header="Image" Command="{Binding AddImageCommand}"/>
                                     <MenuItem Header="Text" Command="{Binding AddSourceCommand}" CommandParameter="{x:Static models:SourceType.TextOverlay}"/>
                                     <MenuItem Header="Countdown" Command="{Binding AddSourceCommand}" CommandParameter="{x:Static models:SourceType.Countdown}"