Tasks

• src/email/envelope.rs: the in_reply_to field, plus parse_message_ids reading the ids off the angle brackets that delimit them and normalising each like message_id.
• src/imap/backend.rs: the 9th ENVELOPE element, at no cost beyond the FETCH the listing already issues.
• src/jmap/backend.rs: inReplyTo added to the Email/get envelope properties and normalised on the way in.
• src/gmail/backend.rs: one more header off the metadata payload.
• src/maildir/backend.rs and src/m2dir/backend.rs: the parsed header, read through both shapes mail-parser yields for a msg-id list.
• src/msgraph/backend.rs: left empty, with the reason (internetMessageHeaders is not in a listing selection).
• src/pimdir/backend.rs: read from the v: 1 meta and written into it by derive_link_and_meta, so a locally added message summarises like a synced one.
• pimdir SPEC Annex A.1: in_reply_to as an optional array of bare msg-ids, additive at v: 1; log entry in the pimdir repository.
• Tests: the several-parents, bracketless, empty and unterminated values, and that an id normalises to the same bytes as the parent's message_id.
• The CHANGELOG entry.
• Fold the delta into cairn/spec/backends.md; write cairn/log/2026-08-16-envelope-in-reply-to.md.