Add anonymised spam benchmark corpus

[dtump]

Summary

• add 20 reviewed, anonymised, non-duplicate spam email fixtures
• add offline parser/privacy regression coverage and opt-in live LLM benchmark logging
• document fixture privacy, minimal OpenAI test configuration, and GPT-5 temperature compatibility

Validation

• .venv/bin/ruff check .
• .venv/bin/ruff format --check .
• .venv/bin/pytest -q