feat: add OxCaml dual-compiler support for js_top_worker and odoc

Add support for building js_top_worker and odoc with both the standard
OCaml 5.4 compiler and the OxCaml 5.2.0+ox compiler using cppo
conditional compilation and dual dune stanzas.

js_top_worker changes:
- Bump dune-project to 3.21 for %{ocaml-config:ox} support
- Add dual library stanzas gated by (enabled_if %{ocaml-config:ox})
- Add cppo guards for OxCaml API differences:
- Compilation_unit.Name.t vs string for persistent loader
- Env.report_error ~level:0 (extra parameter)
- Language_extension.set_universe_and_enable_all (oxcaml-only)
- Unit_info.make ~for_pack_prefix (extra parameter)
- Typemod.type_implementation (extra Compilation_unit arg)
- Gate ppx_deriving_rpc with (not %{ocaml-config:ox})

odoc changes:
- Apply upstream oxcaml PR #1399 (art-w/upstream-oxcaml)
- Bump dune-project to 3.21
- Add dual stanzas in loader, model, xref2, odoc, syntax_highlighter
- Add cppo OXCAML guards for compiler API differences
- Support OxCaml features: modes, layouts, labeled tuples, iarray,
unboxed records, module type strengthening, polymorphic arguments,
call position arguments, Import_info.t, Compilation_unit.t

Verified end-to-end: scrollycode demos generate HTML and the
interactive playground evaluates OCaml code in the browser with
both compiler switches.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>