ROS2 Humble Migration

Migrated from a custom pub/sub messaging layer to ROS2 Humble. All sensor nodes, the gait controller, and the planning layer are now ROS2 nodes.

The custom layer was fine for early prototyping but the tooling ecosystem (rviz2, rosbag2, tf2) makes debugging significantly faster. Worth the rewrite.

The Python nodes are slower than the C++ equivalents would be, but the control loop runs at 100Hz comfortably on the Raspberry Pi 5. Will port to C++ if latency becomes an issue.