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

lib/pantallas/pantalla_bienvenida.dart

@@ -3,6 +3,7 @@ import 'package:provider/provider.dart';
 
 import '../estado/estado_navegacion.dart';
 import '../l10n/gen/app_localizations.dart';
+import '../servicios/servicio_bienvenida.dart';
 import '../tema/pluriwave_theme.dart';
 
 /// WU17: first-run welcome surface (`onboarding-welcome` spec, mockup
@@ -14,13 +15,31 @@ import '../tema/pluriwave_theme.dart';
 ///
 /// A full-screen ROUTE, not a modal `Dialog` — deliberately distinct from
 /// the pre-existing `PluriOnboardingDialog` (an unrelated "what's new"
-/// help-content modal already shown from `app.dart`'s launch flow). This
-/// work unit's own task list and verify command scope to this screen in
-/// isolation; wiring it into the real first-launch flow (i.e. deciding
-/// when `app.dart`/`main.dart` should push it) is not part of WU17.
+/// help-content modal already shown from `app.dart`'s launch flow). Both
+/// surfaces coexist: [mostrarSiProcede] (WU17b) is what actually wires this
+/// screen into the genuine first-launch flow, called from `app.dart` BEFORE
+/// `PluriOnboardingDialog.mostrarSiProcede` on every cold start, so the two
+/// never race — the once-ever welcome resolves first, then the recurring
+/// what's-new dialog runs its own unrelated per-version check exactly as
+/// before.
 class PantallaBienvenida extends StatelessWidget {
   const PantallaBienvenida({super.key});
 
+  static final ServicioBienvenida _servicio = ServicioBienvenida();
+
+  /// WU17b: shows this screen once, on the genuine first launch, then never
+  /// again. Mirrors `PluriOnboardingDialog.mostrarSiProcede`'s shape
+  /// (check-then-show-then-mark-seen) so both first-launch surfaces share
+  /// the same call convention from `app.dart`.
+  static Future<void> mostrarSiProcede(BuildContext context) async {
+    if (!await _servicio.debeMostrarBienvenida()) return;
+    if (!context.mounted) return;
+    await Navigator.of(
+      context,
+    ).push(MaterialPageRoute<void>(builder: (_) => const PantallaBienvenida()));
+    await _servicio.marcarBienvenidaVista();
+  }
+
   @override
   Widget build(BuildContext context) {
     final l10n = AppLocalizations.of(context);