Tasks

• Move io-imap onto stream::Stream and the connect options structs.
• Strip io-imap's own retry layer, keeping the empty-read guard.
• Add ImapStream::stop_retrying and call it from the mailbox watch worker.
• Move io-smtp, io-http, io-gmail, io-jmap, io-msgraph and io-pim-discovery onto the new API.
• Unify io-http on 0.4 across the graph, so one copy is resolved instead of two.
• Point himalaya's patch table at the local checkouts and bump io-pim-discovery to 0.6.
• Check every crate and himalaya itself, plus fmt and clippy.
• Verify against a live Fastmail account: mailboxes, envelopes, search, sort, thread, read, append, store, expunge, SMTP and discovery.
• Changelog entries in every touched crate and in himalaya.