Bidirectional bridge between dune pkg and opam switches

dune pkg stores built packages in _build/.pkg/, invisible to the standard opam switch mechanism that tools like ocamllsp, merlin, and utop rely on. locswijch mirrors those built artifacts into a real opam switch using hard links, so editor tooling works unchanged, and the switch doubles as a backup of dune.lock/ and _build/.pkg/ that survives dune clean. Subcommands: sync (project to switch), restore (switch to project), migrate (opam switch to dune.lock/), and trip (end-to-end round-trip test).