Reference range backend for an atomically published local directory.

A store contains immutable packs/pack-<checksum>.pack and .idx files plus manifest.json. The manifest is replaced atomically only after all content-addressed artifacts are present. read_ranges/2 uses positional reads and never shares or advances a mutable file offset.

publish/2 captures every object the repository stores — packed and loose, reachable or not; it never garbage-collects. Existing packs are copied as-is. When loose objects exist, it feeds their complete object-ID list to the configured Git executable's pack-objects plumbing, adding that new pack beside the existing packs in the published manifest. Publication follows objects/info/alternates transitively; a missing alternate objects directory is a loud error rather than an incomplete published store.

Packing loose objects briefly stages a temporary directory below the source repository's objects/ directory so Git can rename its output on the same filesystem; the directory is always removed afterward. A source that cannot create this staging directory returns {:error, :source_repository_read_only}.

:ok =
  Gitility.ODB.RangeBackend.LocalDirectory.publish(
    "/srv/git/project.git",
    "/srv/gitility/project-packs"
  )

{:ok, supervisor} =
  Gitility.ODB.PackFetch.start_link(
    backend: {Gitility.ODB.RangeBackend.LocalDirectory,
              "/srv/gitility/project-packs"},
    into: {:dir, "/var/cache/gitility/project"}
  )