Renderer: suppress camera headlight when stage contains authored lights

Mirrors usdview behaviour -- when the stage has at least one UsdLux
light the default camera fill-light is disabled so the scene is lit
purely by its authored lights.  StageHasAuthoredLights() early-exits
on the first HasAPI<UsdLuxLightAPI>() hit to keep the per-frame cost
minimal.

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

src/core/UsdSceneRenderer.cpp

@@ -571,7 +571,13 @@ void UsdSceneRenderer::Render(int width, int height)
     // Camera headlight: point light (w=1) positioned at the camera world-origin,
     // transformed by the view-inverse so it tracks the camera each frame.
     // (stageView.py: l.position = cam_pos + (1,); l.transform = frustum.ComputeViewInverse())
-    if (m_ambientLightOnly) {
+    //
+    // The headlight is the *default* fill used only while the stage has no
+    // authored lights.  Once the scene contains real UsdLux lights, suppress it
+    // so the scene is lit purely by those lights (Hydra evaluates them via
+    // enableSceneLights) -- mirrors usdview's "use scene lights when present".
+    const bool stageHasLights = StageHasAuthoredLights();
+    if (m_ambientLightOnly && !stageHasLights) {
         pxr::GfMatrix4d viewInverse = m_viewMatrix.GetInverse();
         pxr::GfVec3d camPos = viewInverse.ExtractTranslation();
 
@@ -1527,6 +1533,20 @@ void UsdSceneRenderer::BuildLightWireframeLines(const pxr::UsdPrim&    lightPrim
     }
 }
 
+// ===========================================================================
+// StageHasAuthoredLights
+// ===========================================================================
+
+bool UsdSceneRenderer::StageHasAuthoredLights() const
+{
+    if (!m_stage) return false;
+    for (const pxr::UsdPrim& prim : m_stage->Traverse()) {
+        if (prim.HasAPI<pxr::UsdLuxLightAPI>())
+            return true;
+    }
+    return false;
+}
+
 // ===========================================================================
 // DrawLightWireframes  (mirrors DrawCameraWireframes)
 // ===========================================================================

src/core/UsdSceneRenderer.h

@@ -247,6 +247,10 @@ private:
                                   double                 scale,
                                   std::vector<float>&    outVerts);
 
+    /// True if the stage contains at least one authored UsdLux light.
+    /// Used to suppress the default camera headlight when real lights exist.
+    bool StageHasAuthoredLights() const;
+
     /// Project a world-space point to absolute screen coordinates (x=imagePosX+pixelX, etc.).
     /// Returns false when the point is behind the camera.
     static bool WorldToScreen(const pxr::GfVec3d&    world,