Add CUDA-aware MPI halo exchange for MPMesh

[Shahrear2000]

Main changes:

• Adds CUDA-aware MPI communication path guarded by CUDA_AWARE_MPI.
• Keeps CPU-GPU deep_copy path as fallback.
• Uses cached metadata and per-neighbor GPU buffers for repeated halo exchanges.

Runtime notes:

• Requires MPICH_GPU_SUPPORT_ENABLED=1 for CUDA-aware MPI on Perlmutter.
• Can disable CUDA-aware path at runtime with POLYMPO_DISABLE_CUDA_AWARE_MPI=1.