0527 — pkg/migrate adopts the three-layer template

Context

After [0526] the migrate door was right but the layout was flat: Controller, configs, and gate methods sat in the pkg/migrate root beside the vocabulary (Migration, sentinels, version consts), with the datastore at pkg/migrate/datastore. Every other domain follows pkg/ -> pkg//controller -> pkg//controller/datastore.

Decision

• pkg/migrate is vocabulary only: Migration + registry Validate, ErrNotRegistered, Min/Max version consts. It imports common and core datastore (Querier appears in Migration’s step-func signatures).
• Controller, ControllerConfig, RunOnce/RunAll, the schema gates, and the version/lock reads move to pkg/migrate/controller; the datastore moves to pkg/migrate/controller/datastore.
• Migration.ToStep becomes the controller-side toStep adapter — it builds a datastore shape, so it cannot stay on the vocabulary type.
• ErrNotRegistered is declared in the vocabulary (sentinel-ownership rule); the datastore’s registrationError maps onto it, and the controller’s re-export var is deleted.

Consequences

• Callers needing only errors.Is / []Migration keep importing pkg/migrate; callers building or holding the door import migratecontroller “pkg/migrate/controller” (topic/worker controllers, admin, systemmanager, CLI, invariantlab, schemagatelab).
• The later internal/ move ([public surface trim]) relocates this shape wholesale; no second restructure needed.