0405 — The topic catalog is keyed (name, schema_version), and constructors require SchemaVersion positionally

Context. With a schema version bump being a new physical topic under the same name, the catalog must hold multiple registered versions of a name and every code path must say which one it means.

Decision. topic (name, schema_version) UNIQUE together. RegisterTopic/GetTopic/AlterTopic/DestroyTopic are all version-addressed; RenameTopic moves every version registered under a name. NewProducer/NewConsumer (and the split consumers) take topic.SchemaVersion as a required, positional, typed constructor parameter, and vulkan topic register/get grow a --schema-version dimension.

Consequences. Version is impossible to omit — there is no default that could silently bind the wrong physical topic. Each version has its own log, id space, and duties; the name is a family label, and rename is the only verb that operates on the whole family.

Superseded by [0618]: the schema version is a message_log column declared by the Message type; the topic catalog is keyed by name alone.