Seeding node for radicle artifacts.
Owns all blob I/O: the persistent iroh-blobs store, serving blobs to
peers, and fetching/exporting/HTTP downloads against that store. The
long-running daemon ([node::run]) listens on a Unix control socket;
the rad-artifact CLI (and other radicle-artifact-client users)
drive it over that socket. Long-running seeding is owned by [node];
the async fetch building blocks live in [fetch].