Switch and verify the RMW middleware implementation used by ROS 2 (Lyrical Luth) nodes via RMW_IMPLEMENTATION

Documented steps

1. Know the default: the default middleware in ROS 2 Lyrical Luth is rmw_fastrtps_cpp (eProsima Fast DDS 3.6.x), per the Lyrical supported-platforms page.
2. Know the Tier 1 alternatives shipped for Lyrical: rmw_connextdds (RTI Connext 7.7.0, all architectures except arm64), rmw_cyclonedds_cpp (Cyclone DDS 11.0.x) and rmw_zenoh_cpp (Zenoh 1.8.0). rmw_fastrtps_dynamic_cpp is Tier 2.
3. Install the implementation you want first, either as extra ROS 2 binary packages or from source with its compile-time dependencies present - see the Install RMW implementations guide.
4. Both C++ and Python nodes read the RMW_IMPLEMENTATION environment variable at runtime to select the middleware.
5. Run a talker on a non-default RMW: `RMW_IMPLEMENTATION=rmw_cyclonedds_cpp ros2 run demo_nodes_cpp talker`.
6. Run the listener with the SAME value in its own terminal: `RMW_IMPLEMENTATION=rmw_cyclonedds_cpp ros2 run demo_nodes_py listener`. Both processes must use the same RMW to communicate.
7. Check what is currently in effect: `printenv | grep RMW_IMPLEMENTATION`. If unset, the distro default (rmw_fastrtps_cpp on Lyrical) is in use.
8. After switching, stop the daemon so CLI tools do not keep using the previous implementation: `ros2 daemon stop`, then rerun e.g. `ros2 node list` with the new RMW_IMPLEMENTATION set.
9. Recognise the documented failure messages when the named implementation is not installed: "Expected RMW implementation identifier of 'rmw_connextdds' but instead found 'rmw_fastrtps_cpp', exiting with 102." (one other implementation present) or "Error getting RMW implementation identifier / RMW implementation not installed (expected identifier of 'rmw_connextdds'), exiting with 1." (none matching).
10. When adding a new RMW to an existing source workspace, rebuild with `--cmake-clean-cache` so dependency detection re-runs and the new RMW package is built.
11. If that rebuild complains that the default RMW implementation changed, either restore the previous default via the RMW_IMPLEMENTATION CMake argument, or delete the build folders of the complaining packages and continue with `--packages-start <package name>`.
12. Official doc: https://docs.ros.org/en/lyrical/How-To-Guides/Working-with-multiple-RMW-implementations.html