Add changelog fragment for zero-wrench fast path

Documents the early-return added to apply_external_force_torque when
force_range and torque_range are both zero, restoring H1/G1/Anymal-C
Velocity-Rough throughput regressed by the WrenchComposer rework in
isaac-sim#5265.

Refs: isaac-sim/IsaacLab-Internal#906, isaac-sim/IsaacLab-Internal#911

Author: AntoineRichard

source/isaaclab/changelog.d/fix-zero-wrench-fast-path.rst

@@ -0,0 +1,12 @@
+Fixed
+^^^^^
+
+* Fixed a per-step performance regression in :func:`~isaaclab.envs.mdp.events.apply_external_force_torque`
+  when the event was configured with all-zero ``force_range`` and ``torque_range`` (a common default
+  for tasks that declare the event term but apply no disturbance). The event was unconditionally
+  sampling zero wrenches and routing them through the dual-buffer ``WrenchComposer`` introduced in
+  PR #5265, paying the full per-step compose-and-apply cost in
+  :meth:`~isaaclab.assets.Articulation.write_data_to_sim` for what is semantically a no-op. The
+  function now returns early when both ranges are exactly zero. This restores the H1, G1, and
+  Anymal-C ``Velocity-Rough`` throughput observed prior to PR #5265. Behaviour for non-zero ranges
+  is unchanged.