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/ui/ViewportTile.cpp

@@ -94,6 +94,7 @@ ViewportTileSettings ViewportTile::GetSettings() const
     s.renderDelegate   = m_renderer.GetCurrentRendererId().GetString();
     s.showGrid         = m_renderer.ShowGrid();
     s.showCameraGuide  = m_renderer.ShowCameraGuide();
+    s.aov              = m_renderer.GetCurrentAov().GetString();
     s.aaEnabled        = m_renderer.GetAAEnabled();
     const pxr::GfVec3f& bg = m_renderer.GetBackgroundColor();
     s.bgColorR = bg[0]; s.bgColorG = bg[1]; s.bgColorB = bg[2];
@@ -110,6 +111,8 @@ void ViewportTile::ApplySettings(const ViewportTileSettings& s)
         m_renderer.SetRendererPlugin(pxr::TfToken(s.renderDelegate));
     m_renderer.SetShowGrid(s.showGrid);
     m_renderer.SetShowCameraGuide(s.showCameraGuide);
+    if (!s.aov.empty())
+        m_renderer.SetCurrentAov(pxr::TfToken(s.aov));
     m_renderer.SetAAEnabled(s.aaEnabled);
     m_renderer.SetBackgroundColor(pxr::GfVec3f(s.bgColorR, s.bgColorG, s.bgColorB));
     m_renderer.SetBBoxMode(static_cast<BBoxMode>(s.bboxMode));
@@ -1008,6 +1011,20 @@ void ViewportTile::RenderCompactToolbar(int tileIndex)
                 ImGui::EndMenu();
             }
 
+            // -- AOV ---------------------------------------------------------
+            {
+                pxr::TfTokenVector aovs = m_renderer.GetRendererAovs();
+                if (ImGui::BeginMenu("AOV", !aovs.empty())) {
+                    pxr::TfToken currentAov = m_renderer.GetCurrentAov();
+                    for (const auto& aov : aovs) {
+                        bool checked = (aov == currentAov);
+                        if (ImGui::MenuItem(aov.GetText(), nullptr, checked) && !checked)
+                            m_renderer.SetCurrentAov(aov);
+                    }
+                    ImGui::EndMenu();
+                }
+            }
+
             // -- Lighting ----------------------------------------------------
             if (ImGui::BeginMenu("Lighting")) {
                 bool ambient = m_renderer.GetAmbientLightOnly();