Add barrier between cudaIpcCloseMemHandle and cudaFree in Buffer::destroy

[yurekami]

Summary

• Add a barrier between closing remote IPC handles and freeing local buffer
• Prevents undefined behavior per CUDA docs for cudaIpcOpenMemHandle
• Ensures all ranks close remote handles before any rank frees its buffer

Background

According to CUDA documentation, it is undefined behavior to cudaFree an exported memory region before cudaIpcCloseMemHandle is called on all processes that opened the handle.

Without this fix, in a multi-GPU scenario:

1. Rank 1 finishes closing remote handles and proceeds to free its local buffer
2. Rank 2 is still trying to close the handle to Rank 1's buffer
3. This is UB since the underlying memory has been freed

Test plan

• Verify Buffer::destroy completes without hangs on multi-GPU systems
• Run stress tests with explicit buffer destruction

Fixes #497

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