Fix USD camera orbit pivot — use scene bbox centroid instead of focus distance

When switching to a USD camera, the orbit pivot was set from
GfCamera::GetFocusDistance() which is a depth-of-field attribute
that defaults to 5 scene units regardless of scene scale.

InitCameraNavigation() now computes the scene bounding box centroid,
projects it onto the camera view ray to get a proper orbit distance,
and calls SetFocalPoint/SetDist to place the pivot correctly.
Added SetDist() setter on ViewportCamera to support this.

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

src/core/ViewportCamera.h

@@ -113,6 +113,14 @@ public:
         m_cameraTransformDirty = true;
     }
 
+    /// Set the orbit distance directly.
+    /// Used when initialising from a USD camera prim to override the camera's
+    /// authored focus distance (which is a DOF attribute, not an orbit radius).
+    void SetDist(double d) {
+        m_dist = d;
+        m_cameraTransformDirty = true;
+    }
+
     /// Returns the current near-clip distance.
     double GetNearClip() const {
         return static_cast<double>(m_camera.GetClippingRange().GetMin());