Name ADR 0013 in the test that defends it

A migration refused when the record and the files disagree is the
property that makes a schema trustworthy months later. The test asserted
it without naming the decision, so an audit of which decisions are
defended could not see it. novox/hq ADR 0017.

internal/store/live_test.go

@@ -199,6 +199,11 @@ func TestASchemaChangeAndItsRecordCommitTogether(t *testing.T) {
 	}
 }
 
+// Defends novox/hq ADR 0013: a schema change is a numbered migration.
+//
+// The property that makes a schema trustworthy months later is not that migrations ran, but that
+// they refuse to run when the record and the files disagree — an edited migration is a schema
+// nobody can reproduce.
 func TestAChangedMigrationIsRefusedAgainstARealRecord(t *testing.T) {
 	// The same refusal as the unit test, against a record PostgreSQL actually kept — which is
 	// what the guard protects, and the unit test can only model.