OptionaltransfersTransfers that outlive their caller (the web's): start admits one at once and throws a CatalogFailure coded duplicate-download for a repo already running; snapshot is the rows to show. A host without them lists the hub library's own tracker.
OptionalcanWhether a write token is available.
OptionaldownloadFetches id into the hub cache; resolves to its snapshot directory.
OptionalpickThe host's automatic model choice (CatalogEntry's directory and id); without it the catalog makes none.
Optionalsignal: AbortSignalOptionalpublish
The Hub side a host may supply: what downloads a repo and what pushes a directory. Without them the catalog does neither.