Add Camera Guide viewport toggle (stageView.py DrawCameraGuides port)

Adds a per-viewport "Camera Guide" toggle (default off) that draws a full
frustum wireframe box (near quad + far quad + 4 connecting edges) in oracle
orange for every non-active UsdGeomCamera prim, matching usdview's
displayCameraOracles behaviour. Uses GfFrustum::ComputeCorners() and the
same 24-index line layout as DrawCameraGuides. Toggle exposed in the
right-click context menu and gear popup; persists to INI as ShowCameraGuide.

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

src/core/UsdSceneRenderer.h

@@ -180,6 +180,9 @@ public:
     bool              ShowGrid()                const { return m_showGrid; }
     void              SetShowGrid(bool show)          { m_showGrid = show; }
 
+    bool              ShowCameraGuide()         const { return m_showCameraGuide; }
+    void              SetShowCameraGuide(bool show)   { m_showCameraGuide = show; }
+
     bool              GetAAEnabled()            const { return m_aaEnabled; }
     void              SetAAEnabled(bool enabled)      { m_aaEnabled = enabled; }
 
@@ -327,6 +330,7 @@ private:
     std::vector<pxr::GfVec4d> m_clipPlanes;
 
     bool            m_showGrid;
+    bool            m_showCameraGuide;
     bool            m_aaEnabled;        ///< GL_LINE_SMOOTH anti-aliasing for all line overlays
     pxr::GfVec3f    m_backgroundColor;