0650 — The migration version reads through the client

Context

vulkan migrate status and the six migrate <scope> up|down leaves read each target’s current schema version before any DDL runs: the direction guard, the no-op check, and the status table all need it. No client verb exposed that read, so the CLI reached under the client with client.Datastore(), built its own migratecontroller.Controller, and composed common.NewTopicOwner from GetTopic rows twice to feed TopicVersion. [0649] had just moved owner resolution into admin’s SystemOwner / TopicOwner / GroupOwner; the CLI copies were the last callers assembling ids by hand outside the library.

Decision

• The version a scope’s tables are at is a read on the scope’s handle: client.System().MigrationVersion(ctx) and client.Topic[T](name).MigrationVersion(ctx), both (int64, error). The name is the migration_log.migration_version column’s, and it keeps its distance from Versioned.SchemaVersion, which versions payloads.
• admin composes each read the way it composes MigrateSystem / MigrateTopic: SystemMigrationVersion resolves SystemOwner then calls the migrate controller’s SystemVersion; TopicMigrationVersion resolves TopicOwner then TopicVersion. Absence surfaces as the existing errors, ErrNotRegistered and ErrTopicNotFound.
• The read returns the bare version. The datastore’s SchemaStateRow also carries the minimum compatible version in force, but nothing on the client or CLI reads it, so it stays internal until a consumer appears.
• The CLI’s migrateTarget holds a name and a version, no owner, and gatherTargets takes only the client. migrate status builds no controller at all. The advisory-lock pre-flight (IsLocked) still builds one from client.Datastore(), as destroy and manager run do for their own reads; exposing the lock probe on the client is a separate decision.

Consequences

• No caller outside the library composes an owner from catalog rows; the three admin resolvers are the only path, and a destroyed topic reads as not-found rather than as a stale id.
• migrate topics and migrate status resolve each topic by name, one catalog read per topic beside the version read. The lists are operator sized, so the extra round trip is accepted over a second id-keyed read path.
• The client guide’s sample and old-verbs table and the migrations guide name the two reads.