0408 — The topic catalog column is named schema_version, accepting the overlap with schema_log.schema_version

Context. The topic catalog’s new version column shares a name with schema_log.schema_version, which is the DB-migration axis owned by pkg/migrate. The topic column is a different axis entirely: the message compatibility epoch a user sets.

Decision. Keep the name schema_version for both. The two columns are never read in the same query and are documented at both definitions.

Consequences. Rejected: disambiguating the topic column (e.g. topic_version) — judged not worth losing the more natural word for what a user actually sets via --schema-version. Readers encountering either column should check which axis they’re on; the definitions carry that note.

Superseded by [0618]: the schema version is a message_log column declared by the Message type; the topic catalog is keyed by name alone.