Add GitHub Actions CI: build, test, docs on OCaml 5

[tiensonqin]

Summary

Adds .github/workflows/ci.yml — the repo previously had no CI. On pushes to main and on PRs, a single build job on ubuntu-latest:

• ocaml/setup-ocaml@v3 with ocaml-compiler: "5" (latest OCaml 5.x) and dune-cache: true; setup-ocaml also caches the opam root/switch automatically.
• opam install . --deps-only --with-test, plus opam install melange odoc — melange is required because src/dune declares (modes native byte melange) and dune-project has (using melange 0.1); odoc is needed for @doc.
• opam exec -- dune build (compiles all three modes), dune runtest, dune build @doc.

Verified locally on OCaml 5.5.1: deps install, full build including the melange mode, 19/19 alcotest tests pass, and @doc builds clean.

Link to Devin session: https://app.devin.ai/sessions/2470dfae4d7d4949971b8a58d50ba0cc
Open in Devin Desktop: https://app.devin.ai/desktop/session/2470dfae4d7d4949971b8a58d50ba0cc?variant=devin
Requested by: @tiensonqin

[devin-ai-integration]

I'll fix CI failures and address comments. I'll skip comments containing "(aside)".

• Disable automatic comment, CI, and merge conflict monitoring

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