feat: add rank0 async HF save logs

[caixianzhang]

Summary

This PR adds lightweight async HF logging in the default path:

• logs foreground async_save_hf launch cost through XTuner profile_time_and_memory
• logs rank0 background async HF writer start/end timing
• logs rank0 writer failure timing before preserving the existing status-file error path

It intentionally does not add per-rank diagnostic JSON, rank0 summary JSON, handle config propagation, or trainer-side diagnostic plumbing.

Validation

• python3 -m py_compile xtuner/v1/engine/train_engine.py xtuner/v1/model/base.py xtuner/v1/model/compose/base.py
• git diff --check

Note: local ruff command is not available in this environment.