OCaml 5.5 / Lwt 6 support

[kit-ty-kate]

Currently dream isn't compatible with OCaml 5.5 due to its reliance to Lwt < 6

[funwithcthulhu]

I started unwinding this and found two separate pieces:

1. Normal install/runtime deps appear to be blocked by the currently published caqti-lwt.2.3.0, which has lwt < 6~. Upstream Caqti v2.3.1 removes that upper bound, but most of the 2.3.1 packages were not yet in opam-repository, so I opened Relax caqti-lwt Lwt upper bound ocaml/opam-repository#29937.

2. With that local opam-repository branch, an isolated opam solver check for Dream non-test deps succeeds with ocaml-base-compiler.5.5.0~beta1 and Lwt 6. It selects caqti.2.3.1, caqti-lwt.2.3.1, lwt.6.1.2, and lwt_ppx.6.1.0.

Full --with-test deps are still blocked separately because dream.opam pulls reason for tests/examples, and the current solver reports reason -> ocaml < 5.5. So the practical path seems to be:

• get Caqti 2.3.1 into opam-repository to unblock normal OCaml 5.5 / Lwt 6 installs;
• then decide whether Dream's OCaml 5.5 CI should initially cover non-test builds only, or wait for the Reason test dependency to support 5.5.

[yawaramin]

Hmm, it’s not clear to me why we should continue to have examples in Reason and/or force the tests to compile them. At this point anyone who wants to use Reason syntax should be able to figure it out themselves…

@Willenbrink what do you think?

[funwithcthulhu]

I opened #430 for the narrower part of this: keep the Reason examples in the tree, but stop Dream's test dependencies and repository-wide Dune build from requiring the Reason compiler. That does not close this issue; the broader OCaml 5.5 / Lwt 6 path still depends on the Caqti/opam side.

[kit-ty-kate]

FYI: Reason with OCaml 5.5 support was released yesterday: https://github.com/reasonml/reason/releases/tag/3.18.0