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The migrator treats version 1 as special when migrating down: the main line's version 1 drops `river_migration`, so there's no row left to delete. That special case applies to every line, though, and a non-main line's version 1 doesn't touch `river_migration`. Migrating such a line down through version 1 leaves its version 1 row in place, and with `MigrateTx` it leaves the rows for every version removed in that run. A later up migration then sees version 1 as still applied and skips it, and another down migration reruns version 1's down SQL. Scope both version 1 special cases (the per-version delete and the batch delete used within an outer transaction) to the main line, so other lines delete their rows like any other version. Extend the alternate line tests to check that migrating fully down removes all of the line's rows and that migrating back up reapplies version 1, add a test that stepping down from version 1 removes its row and leaves nothing more to migrate, and cover the same round trip through `MigrateTx`.
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When a migration line other than
mainis migrated down through its version 1,rivermigrateleaves that version's row inriver_migration. Only the main line's version 1 drops the table, so the "nothing left to delete" shortcut is wrong for other lines. WithMigrateTxthe same shortcut leaves rows for every version removed in the run. Afterwards, a later up migration skips the line's version 1, and a later down migration runs its down SQL a second time.This limits both version 1 shortcuts to the main line, so other lines delete their rows like any other version. New tests cover a full down-and-up round trip on an alternate line, with and without an outer transaction, and stepping down from version 1 one step at a time.