Two short-lived branches can each grab the next free migration number and
both merge, leaving the tree with two directories sharing a four-digit
prefix. This structural test fails the build the moment a prefix repeats, so
the collision surfaces at PR CI instead of after merge. One historical
collision (0025) predates the guard and is grandfathered — a migration
directory name is the primary key in the applied-migrations ledger, so it
cannot be renamed without orphaning the record on every deployed database.