fix(bienvenida): wire the welcome screen into the first-launch flow

test/servicios/servicio_bienvenida_test.dart

@@ -0,0 +1,44 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:pluriwave/servicios/servicio_bienvenida.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+/// WU17b: `ServicioBienvenida` decides whether the first-run welcome
+/// screen (`PantallaBienvenida`) is still due — same injectable-prefs,
+/// versioned-key convention as `ServicioContenidoApp` (S3-R4), but a plain
+/// boolean flag since this welcome is a one-time, not per-version, surface.
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+
+  group('ServicioBienvenida', () {
+    test(
+      'debeMostrarBienvenida is true before it has ever been marked seen',
+      () async {
+        SharedPreferences.setMockInitialValues({});
+        final servicio = ServicioBienvenida();
+
+        expect(await servicio.debeMostrarBienvenida(), isTrue);
+      },
+    );
+
+    test(
+      'debeMostrarBienvenida is false after marcarBienvenidaVista',
+      () async {
+        SharedPreferences.setMockInitialValues({});
+        final servicio = ServicioBienvenida();
+
+        await servicio.marcarBienvenidaVista();
+
+        expect(await servicio.debeMostrarBienvenida(), isFalse);
+      },
+    );
+
+    test('respects a seen flag already persisted by a prior launch', () async {
+      SharedPreferences.setMockInitialValues({
+        'pluri_bienvenida_vista_v1': true,
+      });
+      final servicio = ServicioBienvenida();
+
+      expect(await servicio.debeMostrarBienvenida(), isFalse);
+    });
+  });
+}