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.h

@@ -17,6 +17,19 @@
 
 namespace UsdLayerManager {
 
+/// Serialisable snapshot of all per-tile render settings.
+struct ViewportTileSettings {
+    std::string renderDelegate;           ///< TfToken string, empty = default (Storm)
+    bool        showGrid         = true;
+    bool        aaEnabled        = false;
+    float       bgColorR         = 0.15f;
+    float       bgColorG         = 0.15f;
+    float       bgColorB         = 0.15f;
+    int         bboxMode         = 0;     ///< BBoxMode cast to int
+    bool        ambientLightOnly = false;
+    bool        domeLightEnabled = false;
+};
+
 /// Named orthographic view directions.
 /// When m_orthoView != None the tile renders an orthographic camera locked to
 /// that world-space direction; the free-camera orbital state (center + dist)
@@ -87,6 +100,11 @@ public:
 
     void FrameScene();
 
+    /// Snapshot all current render settings into a portable struct.
+    ViewportTileSettings GetSettings() const;
+    /// Apply a previously-saved settings snapshot (safe to call before first Render).
+    void ApplySettings(const ViewportTileSettings& s);
+
 private:
     // ── Render sub-functions ─────────────────────────────────────────────────
     pxr::GfCamera ResolveCamera();