Versioned metadata records and the immutable image they apply to.
crabka-metadata provides [MetadataRecord] (the versioned union
of topic / partition / broker registrations / topic deletions) and
[MetadataImage] (an immutable snapshot of the cluster's metadata).
The image is mutated only by [MetadataImage::apply] called from
the Raft state machine in crabka-raft. Everywhere else it's read
via shared references and Arc clones.
See the design spec at
docs/superpowers/specs/2026-05-12-crabka-metadata-quorum-design.md.