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>

lidar/Dockerfile

@@ -7,12 +7,12 @@ SHELL ["/bin/bash", "-c"]
 
 RUN apt-get update && apt-get install -y --no-install-recommends \
     python3-colcon-common-extensions \
-    git \
     && rm -rf /var/lib/apt/lists/*
 
 WORKDIR /ws
 
-RUN git clone --depth 1 https://github.com/slamtec/sllidar_ros2.git src/sllidar_ros2
+# Source is managed as a git subtree — copy directly from the workspace
+COPY lidar/sllidar_ros2/ src/sllidar_ros2/
 
 RUN source /opt/ros/${ROS_DISTRO}/setup.bash && \
     colcon build --packages-select sllidar_ros2