Add light shape wireframe display in viewport

Draw a type-appropriate wireframe gizmo for every UsdLux light in the stage,
mirroring the existing camera-wireframe overlay (dedicated VAO/VBO reusing the
bbox shader, drawn on top with depth-test off).

Shapes, built in the light's local space and pushed through its local-to-world
transform:
  - SphereLight   3 orthogonal circles (radius); axis-cross when treatAsPoint
  - RectLight     width x height rectangle + emission direction line
  - DiskLight     circle + direction line
  - CylinderLight two end-cap circles + connectors (radius/length)
  - DistantLight  sun disc + parallel rays
  - DomeLight     3 large display-scaled circles

Selected lights draw accent orange, others warm yellow.

Verified by creating a SphereLight (3-circle sphere) and a RectLight
(rectangle) in-app: distinct shapes, correct selected/deselected colours,
gizmos visible over geometry.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/core/UsdSceneRenderer.h

@@ -108,6 +108,16 @@ public:
         const pxr::GfMatrix4d&     viewProjMatrix,
         double                     viewportCameraDist);
 
+    /// Draw wireframe gizmos for all UsdLux light prims in the stage.
+    /// The shape reflects each light's type (sphere/rect/disk/distant/dome/
+    /// cylinder); selected lights get an accent colour.  viewportCameraDist
+    /// scales direction lines and point/distant/dome markers.
+    void DrawLightWireframes(
+        pxr::UsdStageRefPtr        stage,
+        const pxr::SdfPathVector&  selectedPaths,
+        const pxr::GfMatrix4d&     viewProjMatrix,
+        double                     viewportCameraDist);
+
     /// Test a screen-space mouse position against all camera wireframe segments.
     /// Returns true and sets *outCameraPath if a camera wireframe is within
     /// kCameraPickRadius (10 px) of mousePos.  Prioritises the closest hit.
@@ -217,6 +227,8 @@ private:
     void DestroyBBoxResources();
     void InitCamWireResources();
     void DestroyCamWireResources();
+    void InitLightWireResources();
+    void DestroyLightWireResources();
     /// Draw a single axis-aligned box from a GfRange3d.
     void DrawBox(const pxr::GfRange3d& range, const pxr::GfMatrix4f& mvp);
 
@@ -227,6 +239,14 @@ private:
                                    double                scale,
                                    std::vector<float>&   outVerts);
 
+    /// Build light-gizmo line segments (GL_LINES vertex pairs) in world space for
+    /// a single light prim.  localToWorld is the light's transform at m_currentTime;
+    /// scale is the display-size factor (for direction lines / point markers).
+    void BuildLightWireframeLines(const pxr::UsdPrim&    lightPrim,
+                                  const pxr::GfMatrix4d& localToWorld,
+                                  double                 scale,
+                                  std::vector<float>&    outVerts);
+
     /// 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,
@@ -307,6 +327,10 @@ private:
     GLuint m_camWireVAO  = 0;
     GLuint m_camWireVBO  = 0;   ///< dynamic VBO; reallocated per frame as needed
 
+    // --- Light wireframe GL resources (mirrors camera wireframe) ---
+    GLuint m_lightWireVAO = 0;
+    GLuint m_lightWireVBO = 0;  ///< dynamic VBO; reallocated per frame as needed
+
     // --- Camera wireframe cache ---
     pxr::SdfPathVector m_cachedCameraPaths;
     bool               m_cameraCacheDirty = true;