Add renderer purpose display toggles (Guide, Proxy, Render)

Exposes UsdGeomImageable purpose visibility as three per-viewport toggles
in the gear popup (Purpose: Guide / Proxy / Render), wired to
showGuides/showProxy/showRender on UsdImagingGLRenderParams. Defaults match
the USD render params defaults (Proxy on, Guide and Render off). Settings
persist per-viewport to viewport_settings.ini.

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

src/core/UsdSceneRenderer.h

@@ -185,6 +185,13 @@ public:
     bool              ShowCameraGuide()         const { return m_showCameraGuide; }
     void              SetShowCameraGuide(bool show)   { m_showCameraGuide = show; }
 
+    bool              ShowGuides()              const { return m_showGuides; }
+    void              SetShowGuides(bool show)        { m_showGuides = show; }
+    bool              ShowProxy()               const { return m_showProxy; }
+    void              SetShowProxy(bool show)         { m_showProxy = show; }
+    bool              ShowRender()              const { return m_showRender; }
+    void              SetShowRender(bool show)        { m_showRender = show; }
+
     const pxr::TfToken& GetCurrentAov()        const { return m_currentAov; }
     pxr::TfTokenVector  GetRendererAovs()       const;
     bool                SetCurrentAov(const pxr::TfToken& aov);
@@ -342,6 +349,9 @@ private:
 
     bool            m_showGrid;
     bool            m_showCameraGuide;
+    bool            m_showGuides;
+    bool            m_showProxy;
+    bool            m_showRender;
     pxr::TfToken    m_currentAov;
     bool            m_aaEnabled;        ///< GL_LINE_SMOOTH anti-aliasing for all line overlays
     pxr::GfVec3f    m_backgroundColor;