Fix build warnings and odoc documentation errors
- Add explicit unix library dependency to dune files (html5rw, tomlt)
- Fix odoc heading levels ({0 -> {1) in imap subject.mli and thread.mli
- Fix code block indentation in subject.mli and h2_stream.mli
- Change unresolved module references to plain text (Bytesrw_unix,
Bytesrw_eio, Webfinger.Jrd.t)
- Fix @raise tags to use Error instead of Error.t
- Escape @mention/@mentions text in poe docs to avoid unknown tag warnings
- Remove unreachable flag_perm rule and redundant list_mailbox production
from IMAP grammar
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>