fix: first native launch since refactor crashed — 3 stacked faults, all closed; RoundClip 'known failure' root-caused and green

User launch (2026-09-01 19:07) NRE'd in MainViewModel ctor:
1. SceneGraph (TASK 31) was 'null!'-declared, assigned mid-ctor, but Scenes is touched
   ~120 lines earlier — field-initialized now.
2. LeftPanel extraction (c9fd1bd) moved StaticResource users (EyeButton/EyeIconStyle)
   into a UserControl while the styles stayed window-scope — invisible at parse time;
   moved to Themes/Controls.xaml (app scope, the existing rule). Full audit: these were
   the only two offenders (grep of Controls/*.xaml StaticResource keys vs app dictionary).
3. RoundClipInteractionTests — the second 'known failure' the map never explained: it
   was two stale-test layers (window.FindName across the new UserControl namescope +
   VisualTreeHelper.HitTest, which returned the IsHitTestVisible=False WebViewHostPanel
   overlay for EVERY point; UIElement.InputHitTest — the real input pipeline — shows the
   corner IS grabbable in both Traditional and Round). Test fixed, no product bug.
Verified: clean rebuild 0 warnings; app boots (log shows full MainWindow loaded; user
clicked + closed, zero new exceptions; real DB Webcam row = the genuine C920, untouched);
RoundClip + 6 RealApp classes pass natively per-class.
Docs: ai.md known-failure note → 246/247 (audio only); TASK 31 verification paragraph
corrected ('cannot run headless' overstated — per-class Windows-host vstest runs them);
MyMistakes: InputHitTest-vs-VTH recipe + namescope/app-style + shared-log facts.
Spin-guard citations: WPF Visual Tree Overview (InputHitTest vs VisualTreeHelper hit
semantics) + XAML namescope docs, learn.microsoft.com.

ViewModels/MainViewModel.cs

@@ -41,7 +41,11 @@ public partial class MainViewModel : ViewModelBase
     // Scene graph (TASK 31): owns the scene collection + element inventory and the
     // baked-crust compositor cache. The ViewModel exposes Scenes/StagedScene/LiveScene
     // on top of it; the scene collection itself lives here.
-    private SceneGraph _sceneGraph = null!;
+    // Field-initialized, NOT ctor-assigned: the ctor touches Scenes (CollectionChanged
+    // wiring) before its later statements run — first-ctor-line NRE if the graph isn't
+    // born yet (startup crash regression from TASK 31, 2026-09-01; headless RealApp
+    // tests never ran it, so it waited for the first native launch).
+    private SceneGraph _sceneGraph = new();
     private readonly TransitionService _transition = new();
 
     private WebView2Manager? _webView2Manager;
@@ -294,7 +298,6 @@ public partial class MainViewModel : ViewModelBase
 
         _chatLayer = new ChatOverlayLayer(new ChatBoxRenderer());
 
-        _sceneGraph = new SceneGraph();
         _rtmpUrlProvider = () => _reusableStreamUrl; // TASK 5: the reusable stream's ingest URL
         _framePump = new FramePump(
             sceneProvider: () => StagedScene,