Fix rotate manipulator axis and gizmo pivot placement

Rotate manipulator — correct axis bug:
- Object-space delta axis was set to initRot.GetRow3(axis) (a parent-space
  vector) then applied in the prim's local frame via deltaM * initRot.
  These frames are mismatched: the axis must be the canonical e_a so that
  Rot(e_a) fixes e_a and the prim rotates purely about its own axis.
  Fixed to use the canonical axis for both spaces; only composition order
  differs (deltaM * initRot for object, initRot * deltaM for world).

Gizmo pivot placement:
- ComputeGizmoPivot() used the bounding-box centre (geometry centroid),
  placing the gizmo at the wrong location for any non-centred prim.
  Replaced with parentToWorld.Transform(translate + pivot) from
  XformCommonAPI::GetXformVectors — the authored pivot point in world space.
  Fallback for incompatible op stacks uses worldMatrix.ExtractTranslation().

Property panel:
- Show pivot (X/Y/Z) as read-only row below Scale, populated from
  XformCommonAPI::GetXformVectors each frame.
- Removed active-rotate-axis display (was added and then removed per request).

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

src/ui/TransformManipulator.h

@@ -108,19 +108,15 @@ private:
     //
     // Ring plane: world-space ring normal (for mouse-ray intersection + sign).
     //
-    // Delta-rotation axis differs by TransformSpace:
-    //   Object => initRot.GetRow3(axis)   -- local axis in PARENT space
-    //             (exact usdtweak localPlaneNormal convention)
-    //   World  => world unit vector
-    //             (correct for root-level / simple hierarchies)
-    //
-    // resultingRotation formula:
-    //   Object => GfMatrix4d(1).SetRotate(delta) * initRot
-    //             (usdtweak: delta applied in local frame before initRot)
-    //   World  => initRot * GfMatrix4d(1).SetRotate(delta)
-    //             (USD row-vector: initRot maps local->parent, then world delta)
+    // Delta-rotation axis is the canonical axis e_a for the dragged ring in
+    // BOTH spaces; only the composition order differs:
+    //   Object => SetRotate(delta) * initRot
+    //             delta applied in the prim's own local frame, so the dragged
+    //             local axis stays fixed -> rotates purely about its own axis.
+    //   World  => initRot * SetRotate(delta)
+    //             initRot maps local->parent, then delta about the world axis.
     pxr::GfVec3d    m_dragRotatePlaneNormal;     // world-space
-    pxr::GfVec3d    m_dragRotateDeltaAxis;       // space-dependent (see above)
+    pxr::GfVec3d    m_dragRotateDeltaAxis;       // canonical axis e_a for ring
     pxr::GfVec3d    m_dragRotateFrom;            // clock-hand at drag start
     pxr::GfMatrix4d m_dragRotateInitialRotMat;   // local->parent rot at drag start
     bool            m_dragRotateObjectSpace = true;