Add per-viewport AOV selection (stageView.py GetRendererAovs port)

Exposes the render delegate's available AOVs in the gear popup under a new
"AOV" sub-menu. Selecting an AOV calls SetRendererAov on the engine and
persists the choice per-viewport in viewport_settings.ini. Switching the
render delegate resets the AOV to "color", matching stageView.py's
_handleRendererChanged. Menu is greyed-out when the delegate reports no AOVs.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/core/UsdSceneRenderer.h

@@ -183,6 +183,10 @@ public:
     bool              ShowCameraGuide()         const { return m_showCameraGuide; }
     void              SetShowCameraGuide(bool show)   { m_showCameraGuide = show; }
 
+    const pxr::TfToken& GetCurrentAov()        const { return m_currentAov; }
+    pxr::TfTokenVector  GetRendererAovs()       const;
+    bool                SetCurrentAov(const pxr::TfToken& aov);
+
     bool              GetAAEnabled()            const { return m_aaEnabled; }
     void              SetAAEnabled(bool enabled)      { m_aaEnabled = enabled; }
 
@@ -331,6 +335,7 @@ private:
 
     bool            m_showGrid;
     bool            m_showCameraGuide;
+    pxr::TfToken    m_currentAov;
     bool            m_aaEnabled;        ///< GL_LINE_SMOOTH anti-aliasing for all line overlays
     pxr::GfVec3f    m_backgroundColor;