fix(oxcaml): js_of_ocaml compile whole for parameterised instances

[art-w]

Fixes #12564

Follow up on #12561 : Js_of_ocaml and wasm_of_ocaml don't currently support the incremental compilation of OxCaml parameterised libraries instantiation, so we have to enforce whole program compilation.

[hhugo]

I don't really understand the direction taken by this PR. The change proposed here is not discover-able, hard to explain. The user might not even be aware that there are parameterised instances in the cone of dependencies. I think, time would be better spent fixing the actual issue. I had a quick look and the issue might be because of https://github.com/ocsigen/js_of_ocaml/pull/2105#discussion_r2597995219.

[hhugo]

I've pushed a quick fix in the oxcaml branch in jsoo repo. I believe that what's missing now is a proper setup in dune itself

[art-w]

Thanks @hhugo ! I'll update the PR to enable separate compilation, the whole compilation restriction was only intended as a temporary fix to avoid crashing :)