0342 — A migration is a sparse struct of func fields, run against a Querier that cannot manage transactions

Context. Each migration step needs optional validate/up/down behavior, and the runner must guarantee a step’s DDL and its version row commit atomically — a step that opens its own transaction would break that. Dated approximately; built across July 2026.

Decision. Migration/TopicMigration are sparse structs with func fields (ValidateUp, Up, ValidateDown, Down, plus a NoTxn bool). Steps receive a Querier interface that excludes Begin/Commit/Rollback, making it a compile error for a step to manage its own transaction; the runner owns the boundary and commits Validate + Up/Down + the version row atomically.

Consequences. A plain step needs no empty stubs, and transaction ownership is enforced by the type system rather than by convention. Rejected: an interface for migration steps — every plain step would carry empty stub methods.