Enhance C++ MPC demo fidelity with LQR rollout

[Kimdooly]

Summary

• add Eigen dependency to the C++ MPC package
• replace the minimal demo with a richer LQR-style MPC rollout that mirrors the Python behavior and publishes predictions on race stack topics
• keep publishing the existing drive, state, pose, and predicted path markers while integrating the commanded state each cycle
• ship launch and parameter assets (plus README notes) so the C++ MPC node can run immediately with --symlink-install

Testing

• Not run (not requested)

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Codex Task