Add isaaclab changelog fragment for the new indexed-fabric Warp kernels

This branch adds four new public Warp kernels to
``isaaclab.utils.warp.fabric`` (the indexed compose/decompose pair plus
the two local↔world propagation kernels) but had no fragment for the
``isaaclab`` package.  CI's "Verify changelog fragments" check fails the
PR without one — every ``source/<pkg>/`` touched needs its own fragment.

Author: pv-nvidia

source/isaaclab/changelog.d/pv-5381-rebased.rst

@@ -0,0 +1,18 @@
+Added
+^^^^^
+
+* Added :func:`~isaaclab.utils.warp.fabric.decompose_indexed_fabric_transforms`
+  and :func:`~isaaclab.utils.warp.fabric.compose_indexed_fabric_transforms`
+  Warp kernels.  They mirror the existing
+  ``decompose_fabric_transformation_matrix_to_warp_arrays`` /
+  ``compose_fabric_transformation_matrix_from_warp_arrays`` kernels but
+  operate on :class:`wp.indexedfabricarray`, so the view-to-fabric mapping
+  is baked into the array and the kernel just dereferences
+  ``ifa[view_index]`` instead of taking a separate ``mapping`` argument.
+* Added :func:`~isaaclab.utils.warp.fabric.update_indexed_local_matrix_from_world`
+  and :func:`~isaaclab.utils.warp.fabric.update_indexed_world_matrix_from_local`
+  Warp kernels that propagate ``local = world * inv(parent)`` and
+  ``world = local * parent`` directly on Fabric storage matrices (no
+  explicit transposes).  Used by
+  :class:`~isaaclab_physx.sim.views.FabricFrameView` to keep child world and
+  local matrices consistent across writes without round-tripping through USD.