PR: rtabmap_ros Gazebo demo: FusionCore + icp_odometry feedback loop (Option A)

[manankharwar]

Context

matlabbe (rtabmap maintainer) approved adding a FusionCore demo to rtabmap_ros.
Tracking issue: introlab/rtabmap_ros#1418

What to build

A complete TurtleBot3 Gazebo-classic demo in the rtabmap_demos package showing the Option A feedback loop:

FusionCore (wheels + IMU)
    → publishes odom frame
        → icp_odometry uses guess_frame_id: odom
            → icp_odom feeds back into FusionCore via encoder2
                → FusionCore outputs tighter /odometry/filtered
                    → loop continues

PR contents

• Copy existing TB3 demo from rtabmap_ros/rtabmap_demos as base
• FusionCore as LifecycleNode with automatic configure/activate transitions
• icp_odometry config: guess_frame_id: odom, publish_tf: false, publish_null_when_lost: false
• rtabmap node: Odom/ResetCountdown: 1 for auto-recovery
• FusionCore YAML config tuned for TB3 sensor set
• Short README explaining the feedback loop architecture

Target

PR against introlab/rtabmap_ros ros2 branch.
Link back to introlab/rtabmap_ros#1418 in the PR.