Sync opam package metadata including x-maintenance-intent
and external dependency specifications.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

- 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>

- ocaml-net/lib/http.ml: Fix Eio.Flow type compatibility for cohttp-eio
- ocaml-net/fuzz/fuzz_net.ml: Fix unused variable warning
- kdf/*/tests/dune: Add (modes native) to avoid js_of_ocaml requirement
- ocaml-matter/dune-project: Remove pins (packages in monorepo)
- Update .ocamlformat versions to 0.28.1 across packages
- ocaml-meross: Various improvements

- Replace local hex_of_string/string_of_hex with ohex in tests:
- ocaml-spake2/test/test_spake2.ml
- ocaml-matter/test/test_tlv.ml
- ocaml-cbort/test/test_cbort.ml

- Make pase.ml re-export Crypto.sha256/hmac_sha256 instead of duplicating

- Remove odoc-xo rules from ocaml-yamlrw/doc/dune

- Apply ocamlformat to yamlrw, yamlt, bytesrw-eio