Add collision to the car in Visual lander worlds

[anshuman83-40]

Fixes #797

Problem

The car in visual_lander.world and visual_lander_circuit.world is an <actor>. Gazebo Harmonic's physics system ignores all links that belong to actors (gz-sim8 Physics.cc: // ignore links from actors for now), so adding a collision to the actor would not help: the drone falls through the car.

Changes

• New system plugin waypoint_follower_harmonic (CustomRobots/waypoint_follower_harmonic), built from CustomRobots/CMakeLists.txt the same way as follow_person_harmonic. It takes the same <waypoint><time/><pose/></waypoint> list as an actor trajectory, supports <loop>, and moves the model using model velocity commands (not teleporting), so a drone that has landed on the car is carried along. If the model is more than teleport_distance (default 1 m) off its path, e.g. after a reset, it is moved straight back.
• Both Visual Lander worlds: the car actor is replaced by a car model that uses the same car_color_beacon mesh and the same waypoints, so the route and timing are unchanged. It has two collision boxes sized from the mesh: the body (kept 0.3 m above the ground so it does not drag) and the roof, whose top is about 2.04 m above the ground, where the drone lands.

The car_color_beacon model itself is not modified.

• Launchers: visual_lander.launch.py and visual_lander_circuit.launch.py append custom_robots' lib folder to GZ_SIM_SYSTEM_PLUGIN_PATH (resolved with get_package_prefix), as other launchers do for their plugins.

Headless test

Ran on Ubuntu 22.04 + ROS 2 Humble + Gazebo Harmonic: colcon build of custom_robots, plugin path taken from the launch files, then each world run headless with a 1.5 kg box (drone stand-in) dropped on the car roof. Run: https://github.com/anshuman83-40/RoboticsInfrastructure/actions/runs/36816877508

World	Result
visual_lander.world (this PR)	Box rests on the roof (z ≈ 2.14 m); car moved 14.10 m, box moved 14.13 m with it
visual_lander_circuit.world (this PR)	Box rests on the roof (z ≈ 2.14 m); car moved 16.78 m, box moved 16.76 m with it
visual_lander.world (current humble-devel, actor)	Box falls through the car to the ground (z ≈ 0.15 m)

The plugin builds without compiler warnings, and the server logs show no plugin errors.

Status

Draft until the before/after videos recorded in Robotics Academy are added.