Decode each body part once (remove double decode)

[kurok]

Finding (Medium). Text parts are decoded twice — once via get_body_raw() and again via get_body() (mail_parser.rs:53–65). Storing the raw bytes once and deriving the string removes ~10–20% of body-decode overhead.

Acceptance

• get_body_raw() is called once per part
• Benchmark shows no regression (likely a gain)

Audit ref: M2-T3 (do after the classifier rewrite).