[codex] Add Gazebo navigation demo integration

[rsasaki0109]

Summary

• switch path_planner_node to build A* state from /map and use /odom for robot pose
• add a Gazebo navigation launch flow for TurtleBot3, including the extra /cmd_vel Twist bridge required by the Rust DWA node
• reduce ROS node log noise and document the end-to-end demo with screenshot and mp4 assets

Why

The ROS2 nodes were individually runnable, but the workspace was missing a repeatable full-stack Gazebo demo that connected SLAM, global planning, local planning, and TurtleBot3 actuation. This change makes the integrated navigation path reproducible and easier to validate.

User Impact

• ./ros2_nodes/launch/run_gazebo_demo.sh now launches the complete demo stack
• /planned_path is published with transient-local durability so late subscribers can inspect it reliably
• the docs now include the working launch sequence, troubleshooting notes, and demo media

Validation

• cargo build --release --manifest-path ros2_nodes/path_planner_node/Cargo.toml
• cargo build --release --manifest-path ros2_nodes/dwa_planner_node/Cargo.toml
• cargo build --release --manifest-path ros2_nodes/slam_node/Cargo.toml
• python3 -m py_compile ros2_nodes/launch/navigation_demo.launch.py
• bash -n ros2_nodes/launch/run_gazebo_demo.sh
• launched Gazebo with ROS_DOMAIN_ID=42 and verified SLAM -> A* -> DWA -> /cmd_vel using /goal_pose = (0.5, 0.0)
• recorded and checked docs/gazebo_demo.mp4 with ffprobe