Add per-viewport render settings persistence (delegate, grid, AA, bg, bbox, lighting)

ViewportTileSettings struct captures: render delegate, show grid, AA, background
color, bbox mode, ambient-light-only, dome-light flags.

ViewportPanel::SaveSettings() writes a simple INI file (layout + one [TileN]
section per tile) to %APPDATA%\UsdLayerManager\viewport_settings.ini on
Application::Shutdown().

ViewportPanel::LoadSettings() is called at the end of Application::Initialize()
— after the viewport panel is constructed but before the first frame — so the
saved render delegate is picked up by UsdSceneRenderer::InitRenderer() on the
first Render() call.  Layout (Single/HSplit/VSplit/Quad) and split ratios are
also restored, creating the correct number of tiles before settings are applied.

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

src/ui/ViewportTile.cpp

@@ -87,6 +87,33 @@ void ViewportTile::FrameScene()
     }
 }
 
+// ---------------------------------------------------------------------------
+ViewportTileSettings ViewportTile::GetSettings() const
+{
+    ViewportTileSettings s;
+    s.renderDelegate   = m_renderer.GetCurrentRendererId().GetString();
+    s.showGrid         = m_renderer.ShowGrid();
+    s.aaEnabled        = m_renderer.GetAAEnabled();
+    const pxr::GfVec3f& bg = m_renderer.GetBackgroundColor();
+    s.bgColorR = bg[0]; s.bgColorG = bg[1]; s.bgColorB = bg[2];
+    s.bboxMode         = static_cast<int>(m_renderer.GetBBoxMode());
+    s.ambientLightOnly = m_renderer.GetAmbientLightOnly();
+    s.domeLightEnabled = m_renderer.GetDomeLightEnabled();
+    return s;
+}
+
+void ViewportTile::ApplySettings(const ViewportTileSettings& s)
+{
+    if (!s.renderDelegate.empty())
+        m_renderer.SetRendererPlugin(pxr::TfToken(s.renderDelegate));
+    m_renderer.SetShowGrid(s.showGrid);
+    m_renderer.SetAAEnabled(s.aaEnabled);
+    m_renderer.SetBackgroundColor(pxr::GfVec3f(s.bgColorR, s.bgColorG, s.bgColorB));
+    m_renderer.SetBBoxMode(static_cast<BBoxMode>(s.bboxMode));
+    m_renderer.SetAmbientLightOnly(s.ambientLightOnly);
+    m_renderer.SetDomeLightEnabled(s.domeLightEnabled);
+}
+
 // ---------------------------------------------------------------------------
 void ViewportTile::RefreshCameraList()
 {