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.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,