Add dune-pkg image variant with compiler variants Split the pipeline into independent opam and dune-pkg paths over shared helpers (Distro_util). Add an ocaml-base variant for `dune pkg` builds: ships dune + a baked relocatable compiler, no opam/switch/repo. Scoped to x86_64, apk/apt distros, OCaml >= 5.5. See docs/adr/0001, 0002. On tier-1 distros also build compiler variants (+flambda, +afl, +no-flat-float-array) via the upstream ocaml-options-only-* packages, mirroring how the opam images install variants.