feat: Gazebo/ROS2 showcase

[MiaoDX]

Context

Gazebo is the easiest entry point for ROS users. Capture Gazebo rendered images via ROS topic + get state via TF.

Priority: Do later — after showcase repo exists.

Scope

• Create ros2-gazebo/ directory in roboharness/showcase
• Capture Gazebo rendered images via ROS topic
• Get robot state via TF
• Produce checkpoint screenshots + structured JSON

Dependencies

• feat: create roboharness/showcase repository #139 (showcase repo)

References

• Strategy: docs/ecosystem-strategy.md §ROS Community