Local code execution via AMCL teardown race

Use-after-free in Nav2 for ROS 2 Humble allows local users to execute code by racing initial-pose messages against AMCL shutdown. The nav2_amcl lifecycle teardown resets initial_pose_sub_ while its executor callback can remain active, then frees pf_, allowing handleInitialPose() to access released heap memory. Exploitation requires local access to publish repeated /initial_pose messages while the AMCL node shuts down; the resulting memory corruption can crash the process or execute code in its context.