[codex] Add GPU priority-aware graph-neural MPPI demo

[rsasaki0109]

Summary

• add a GPU priority-aware graph-neural MPPI demo for 48 robots crossing a shared interaction zone
• compare coordinated graph baseline against learned right-of-way messages plus priority-ordered rollout arbitration
• document the new target, benchmark metrics, and GIF in the README gallery

Results

• cross-route collisions: 261 -> 245
• reach basin: 36/48 -> 40/48
• deadlocks: 1 -> 0
• terminal error: 1.97 -> 1.65
• GPU rollout evaluation: 7.753 ms, 2870.5x vs CPU equivalent rollout evaluation

Validation

• cmake --build build --target gpu_priority_graph_neural_mppi -j$(nproc)
• ./bin/gpu_priority_graph_neural_mppi
• ctest --test-dir build --output-on-failure --label-regex 'cpu|python|gpu' -j$(nproc)