Maildir custom keywords

Maildir accounts gained maildir.keywords.dovecot and maildir.keywords.header, both off by default. The first resolves the lowercase info-section slot letters through each mailbox's own dovecot-keywords file, the second reads keywords from X-Keywords or X-Label. With both off the flag set is what it was.

Custom keywords were invisible: the read path mapped the six standard letters and dropped every other character, so envelope list "flag NonJunk" matched nothing where the same search works on IMAP, JMAP and Graph. The shared model has carried custom keywords all along, and every other backend reads them back through Flag::from_raw. Maildir has no single keyword convention, and a slot letter means nothing without the sidecar defining it, so the mechanism is named rather than guessed.

The resolution itself is io-maildir's, not ours. Its client already held dovecot_keywords and keywords_header and honoured them on store while ignoring both on read, which is why this looked like a himalaya feature at all. Its read paths now take the Maildir the entries were listed from and hand back entries carrying resolved flags (MaildirFullEntry::flags), the composition being an I/O-free MaildirFlags::with_keywords the client feeds a table it loads once per call. Here that leaves the config surface, the two assignments handing it to the client, and flag_from_maildir; parse_filename_flags, flag_from_char and the local sidecar loading are gone, and with them a copy of the filename format that could drift from the library's.

Deliberately not done: no CLI surface for writing a custom keyword, FlagArg being a closed four-variant enum, so this is read parity rather than a round trip. One consequence is documented rather than fixed: since no command can name a keyword, flag set replaces the whole set and drops whatever the message carried.

Unreleased dependency: both halves of the io-maildir work are untagged, so Cargo.toml requires io-maildir = "0.3", prepared but not published, with a [patch.crates-io] block pointing at the local checkout until it is. The other half is pimalaya/io-maildir#3, merged but untagged: on 0.2.1 every flag store discards the slot letters, message read --seen included. himalaya-tui reads entries through the same API and needs the same bump.

Verified: build, fmt and clippy clean; 112 tests pass here, seven that were really testing io-maildir having moved upstream, where 80 unit tests (six new) and 15 integration tests (seven new, in tests/keyword_reads.rs) pass alongside 15 doc-tests. The one clippy warning (src/wizard/search.rs:368) is pre-existing. The three reduced feature builds pass, the ones catching a backend crate leaking into the config schema. Contributed as pimalaya/himalaya#735, whose end-to-end run against a throwaway Maildir had envelope search flag NonJunk go from no matches to one; not re-run after the refactor, and not verified against a live dovecot-written mailbox.

Spec updated: backends (ADDED: Maildir surfaces custom keywords on demand; MODIFIED: Local storage backends).