docs: clarify MOSS speaker-label boundaries

[LauraGPT]

Summary

• describe spkS01 / spkS02 as anonymous labels within the current recording
• clarify that MOSS diarization does not identify known people or verify enrolled voiceprints
• retain third-party OpenMOSS attribution, vLLM serving, segment timestamp, and no-external-VAD boundaries
• add a bilingual README regression contract

Verification

• red phase: the new boundary test failed against exact base 3691a4d
• 6 passed in tests/test_funasr_requirement.py
• git diff --check passed

Exact signed+DCO head: aa89e16.

This PR does not close a user issue.