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/ui/ViewportTile.cpp

@@ -93,6 +93,7 @@ ViewportTileSettings ViewportTile::GetSettings() const
     ViewportTileSettings s;
     s.renderDelegate   = m_renderer.GetCurrentRendererId().GetString();
     s.showGrid         = m_renderer.ShowGrid();
+    s.showCameraGuide  = m_renderer.ShowCameraGuide();
     s.aaEnabled        = m_renderer.GetAAEnabled();
     const pxr::GfVec3f& bg = m_renderer.GetBackgroundColor();
     s.bgColorR = bg[0]; s.bgColorG = bg[1]; s.bgColorB = bg[2];
@@ -108,6 +109,7 @@ void ViewportTile::ApplySettings(const ViewportTileSettings& s)
     if (!s.renderDelegate.empty())
         m_renderer.SetRendererPlugin(pxr::TfToken(s.renderDelegate));
     m_renderer.SetShowGrid(s.showGrid);
+    m_renderer.SetShowCameraGuide(s.showCameraGuide);
     m_renderer.SetAAEnabled(s.aaEnabled);
     m_renderer.SetBackgroundColor(pxr::GfVec3f(s.bgColorR, s.bgColorG, s.bgColorB));
     m_renderer.SetBBoxMode(static_cast<BBoxMode>(s.bboxMode));
@@ -738,6 +740,10 @@ void ViewportTile::RenderContextMenu(int tileIndex)
         if (ImGui::MenuItem("Show Grid", nullptr, &showGrid))
             m_renderer.SetShowGrid(showGrid);
 
+        bool showGuide = m_renderer.ShowCameraGuide();
+        if (ImGui::MenuItem("Camera Guide", nullptr, &showGuide))
+            m_renderer.SetShowCameraGuide(showGuide);
+
         ImGui::Separator();
 
         bool ambientOnly = m_renderer.GetAmbientLightOnly();
@@ -1024,6 +1030,10 @@ void ViewportTile::RenderCompactToolbar(int tileIndex)
                 if (ImGui::MenuItem("Show Grid", nullptr, grid))
                     m_renderer.SetShowGrid(!grid);
 
+                bool guide = m_renderer.ShowCameraGuide();
+                if (ImGui::MenuItem("Camera Guide", nullptr, guide))
+                    m_renderer.SetShowCameraGuide(!guide);
+
                 bool aaItem = m_renderer.GetAAEnabled();
                 if (ImGui::MenuItem("Anti-Aliasing", nullptr, aaItem))
                     m_renderer.SetAAEnabled(!aaItem);