feat(t225): freeze Go ERP protocol contract

backend-api/internal/domain/freight_source_errors.go

@@ -0,0 +1,13 @@
+package domain
+
+import "errors"
+
+// Freight source errors are intentionally transport-neutral. The use case maps
+// them to stable sync outcomes without exposing ERP responses or credentials.
+var (
+	ErrFreightSourceNotConfigured = errors.New("freight source is not configured")
+	ErrFreightSourceSessionNeeded = errors.New("freight source session is required")
+	ErrFreightSourceNotFound      = errors.New("freight source order not found")
+	ErrFreightSourceUnavailable   = errors.New("freight source is unavailable")
+	ErrFreightSourceProtocol      = errors.New("freight source protocol is invalid")
+)