Module attachments 

Real MIME/multipart attachment extraction for gmail sync --extract-attachments.

Deliberately separate from crate::gmail::raw_message’s extract_attachment_filenames heuristic scanner: writing attachment bytes to disk needs real multipart boundary parsing and Content-Transfer-Encoding decoding, which a line-oriented header scan can’t provide. This is the only module in the crate that pulls in a full MIME parser (mail-parser), and it’s only ever reached when --extract-attachments is set — the manifest’s attachment_count/ attachment_filenames fields keep coming from the cheap heuristic regardless, by design (see ADR-0065).