Snapshot Conversion — Moved to openalexSnapshot

These functions have moved.

snapshot_to_parquet(), build_corpus_index(), and lookup_by_id() are now part of the openalexSnapshot package.

Please install openalexSnapshot and refer to its documentation for converting the OpenAlex bulk snapshot to Parquet, building ID lookup indexes, and extracting records by ID.

They are no longer present in openalexPro even as error-raising stubs: an exported stub masks the real function when both packages are attached.

# Install openalexSnapshot (once available on r-universe):
pak::pak("openalexSnapshot")

Calling these functions in openalexPro raises an informative error pointing to openalexSnapshot.