Repairs trainer_lookup.source_trainer (discipline-aware). For every already-migrated trainer_lookup row — from the very first to the most recent — this re-reads the current source row, translates each FK below through the current parent mappings, and corrects any column that disagrees.

Update-only: a parent that was never migrated, or a source id of 0/-1, is left alone — nothing is created or auto-copied. A value that is any valid mapping for its parent is treated as correct. The same rule is applied to the newest rows by Mapping FK Sweep.

• trainer_lookup → trainer discipline-aware SourceTrainer → trainer_lookup.source_trainer

Preview runs the exact same reads and resolves the exact same expected values, but skips the write. Every run — preview or real — produces a downloadable Excel report of the changes found.