Manage sllidar_ros2 as a git subtree and apply QoS fix

- Add m5p3nc3r/sllidar_ros2 fork as a git subtree at lidar/sllidar_ros2/
- Dockerfile now COPYs source from the workspace instead of cloning at
  build time; git is no longer a build dependency
- Set /scan publisher QoS to BEST_EFFORT (depth 2) for Nav2/RViz2 compat
- Add lidar/README.md documenting the subtree workflow
- Remove lidar/sllidar_ros2/ from .gitignore (no longer needed)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -23,6 +23,7 @@ services:
       context: .
       dockerfile: lidar/Dockerfile
       platforms:
+        - linux/amd64
         - linux/arm64
     image: raspbot_v2_lidar:latest
     network_mode: host