Validate HcclCommConfig size against CANN/HCCL headers

[morluto]

Summary

checkpoint_engine/distributed/vllm_hccl.py defines HcclCommConfig as a Python ctypes.Structure and passes size=312 when creating sub-communicators. The current Python structure has a larger ctypes.sizeof(...) than 312, but it is not clear whether HCCL expects the full Python mirror or a 312-byte prefix for the CANN/HCCL version this project targets.

Why this matters

If the size field is meant to match the exact C struct size, passing 312 could make the HCCL side reject the config or ignore/misread trailing fields. If 312 intentionally matches an older/prefix ABI, the Python fields after that boundary should be documented or adjusted to avoid future accidental changes.

Suggested validation

• Compare the Python HcclCommConfig layout with the exact CANN/HCCL header version used by supported deployments.
• Confirm whether size=312 is intentional.
• If intentional, document why the Python struct may be larger than the advertised size.
• If not intentional, update the struct or derive size from the validated layout.

Related context

PR #92 fixes misspelled HCCL field names but intentionally does not change the size field because this needs ABI validation against the target HCCL headers.