OxCaml compat: cppo preprocessing, version guards, and new exercises
- Add cppo preprocessing for merlin-js and x-ocaml workers to support
conditional compilation with OXCAML flag
- Guard day10 packages with enabled_if >= 5.3.0 since they need recent OCaml
- Remove fatal odoc warnings from dune-workspace (handled per-package now)
- Bump merlin-js dune lang to 3.17
- Add warning suppression flags where needed (-w -58, -w -67)
- Add interactive extension exercise pages (FOCS 2020/2024/2025, OxCaml
stack allocation)
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>