Webcam resource validation + first-frame proof + CameraConflictProbe: MediaCaptureFrameSource validates post-init (VideoDeviceId match, stream properties, reader StartAsync status), subscribes Failed/CameraStreamStateChanged → SourceFailed; CameraManager.AcquireAsync requires first-frame proof (4s timeout) — silent empty box impossible; MainViewModel surfaces WebcamError chip + MessageBox with suspect-app names; fallback ladder VideoPreview → VideoRecord; 19041 SDK projection gaps documented; AGENTS.md + schema.md + HANDOFF.md memory-map rules added; 81 tests passing, 0 warnings

MainWindow.xaml

@@ -658,6 +658,12 @@
                                 Background="#CC16213e" CornerRadius="4" Padding="8,3" IsHitTestVisible="False">
                             <TextBlock Text="{Binding ResolutionBadgeText}" Foreground="#e0e0e0" FontSize="12"/>
                         </Border>
+                        <Border HorizontalAlignment="Left" VerticalAlignment="Top" Margin="8,8,0,0"
+                                Background="#CCe94560" CornerRadius="4" Padding="8,3" IsHitTestVisible="False"
+                                Visibility="{Binding WebcamError, Converter={StaticResource NotNullToVis}}">
+                            <TextBlock Text="{Binding WebcamError}" Foreground="White" FontSize="12"
+                                       MaxWidth="520" TextTrimming="CharacterEllipsis"/>
+                        </Border>
                         <TextBlock Text="Preview" Foreground="#333" FontSize="24"
                                    HorizontalAlignment="Center" VerticalAlignment="Center"
                                    Visibility="{Binding ShowPreviewPlaceholder, Converter={StaticResource BoolToVis}}"/>