Tidy PyToBytes into a helper; document data models and attachments behavior

[kurok]

Three non-breaking tidy/documentation changes. No public behavior or API change; the full test suite passes unchanged.

Changes

• Replace single-use PyToBytes trait with a private helper #29 (tech-debt): Replace the single-use PyToBytes trait (implemented only for Py<PyAny>, called once in parse_email) with a plain private payload_to_bytes(&Py<PyAny>, Python<'_>) helper. Identical logic: bytes used as-is, str decoded as its UTF-8 bytes, any other type raises TypeError.
• Document attachments-contains-all-parts behavior; add Rust doc-comments #53 (docs): Document that PyMail.attachments contains every node of the MIME tree -- text parts and the multipart container nodes themselves (containers have empty content) -- not just file attachments. Documents existing behavior; nothing changed.
• Document the two parallel data models (PyO3-free core split) #54 (docs/tech-debt): Add module-level doc-comments explaining the two parallel data models: src/mail_parser.rs is the PyO3-free core (Mail/Attachment, independently testable) and src/fast_mail_parser.rs is the PyO3 binding layer (PyMail/PyAttachment). Notes why the split exists (keeps parsing decoupled from the Python bindings).

Verification

• cargo fmt --all -- --check passes
• cargo clippy --all-targets -- -D warnings -W clippy::cast_possible_truncation clean
• pytest -q --ignore tests/benchmark tests: 91 passed

Closes #29
Closes #53
Closes #54