Extend federation manifest schema for url sources with auth and TTL metadata.
Fetch remote capability indexes over HTTP(S), cache under
registry/federation/cache/, and fall back to stale cache on fetch failure.
Add --refresh flag, seven federation tests, and updated federation docs.
Add federation manifest and schema, federation compose and graph CLI commands,
relation cycle/reference checks, federated index and Mermaid graph artifacts,
RegistryFederation guide, and CI validation updates.
