refactor(oci): renames, auth discovery, test convention, docs
- Rename create_client to client, get_blob to blob, get_root_manifest
to root_manifest, get_manifest to manifest (E331)
- Add per-module log source for oci.fetch
- Replace hardcoded Docker Hub URLs with dynamic auth discovery via
/v2/ endpoint WWW-Authenticate challenge
- Apply E600 test module convention (suite as tuple, test.ml entry point)
- Fix test_descriptor to use Alcotest.test_case instead of raw triples
- Remove unused bytes_to_size from checkout.ml
- Add documentation to public interfaces (E400/E405)
- Update uniboot call site for Fetch.client rename