Exclude seeded account revision from migration target occupancy

Why: U3 seeds a directory revision in every new target database, which made an empty PostgreSQL target look occupied to the migration guard.

Evidence: Isolated PostgreSQL cmd/storage-migrate integration tests passed after the previously failing cases reproduced the error.

Impact: A fresh target remains eligible for migration; existing business rows and foreign migration identities still block it.

Refs: IAM U3 Stage 7; docs/proposals/2026-10-14-iam-u3-account-contract-and-plan.md §10

cmd/storage-migrate/main.go

@@ -451,6 +451,11 @@ func guardIdentity(db *sql.DB, migrationID, snapID string, srcVersion int) error
 func businessRows(db *sql.DB) (int64, error) {
 	var total int64
 	for _, t := range copyOrder {
+		// Schema setup seeds this singleton before a migration starts. It is
+		// replaced from the source during copying and is not target business data.
+		if t == "account_directory_revision" {
+			continue
+		}
 		var n int64
 		if err := db.QueryRow(`SELECT COUNT(*) FROM "` + t + `"`).Scan(&n); err != nil {
 			if strings.Contains(err.Error(), "does not exist") {