feat(iap): add freemium unlock via one-time in-app purchase

Adds a permanent, non-consumable premium unlock (EstadoEntitlement +
PuertoCompras/ServicioComprasPlayBilling) that removes ads and unlocks
alarm vacations, alarms past a 5-alarm free cap, recording start, and
full Android Auto browsing. The phone equalizer stays free for everyone.

- Entitlement is prefs-backed (compra_premium_v1), fail-open, and
  resolvable headlessly via esPremiumPersistido() for the Android Auto
  audio handler, which registers before runApp.
- Android Auto reduced mode keeps the real root folder labels for free
  users; browsing into any of them (and playFromMediaId/playFromSearch/
  skipToNext/skipToPrevious) is blocked at the getChildren/servicio_audio
  choke points, with a locked "Función Premium" item as the backstop.
  Current-station play/pause/stop stays untouched. A free -> premium
  transition actively invalidates the head unit's cached browse tree.
- Ads (top banner + capped interstitial before adding a station or an
  alarm) are gated behind entitlement via ServicioAnuncios, using
  official Google test ad unit IDs pending AdMob provisioning.
- Alarm cap UX shows an explanatory message with a secondary unlock
  action rather than a bare paywall jump; existing data is grandfathered.
- 4 new localization keys translated across all 13 supported locales.

Co-located tests use strict TDD (RED test before implementation) for
every new pure-logic unit; full existing suite passes unchanged.

test/widgets/banner_anuncio_superior_test.dart

@@ -0,0 +1,75 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:pluriwave/estado/estado_entitlement.dart';
+import 'package:pluriwave/servicios/servicio_anuncios.dart';
+import 'package:pluriwave/widgets/banner_anuncio_superior.dart';
+import 'package:provider/provider.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+/// Ad-display spec "Persistent Top Banner, Never Overlapping Content": the
+/// banner is entitlement-aware and reserves layout via a `Column`
+/// (`SizedBox.shrink()` collapses it to zero footprint) — never a `Stack`
+/// overlay. AdMob's own `BannerAd.load()` cannot reach a real ad server in
+/// `flutter test` (no plugin channel registered), so it always resolves to
+/// "unloaded" here — exactly the same degrade-to-shrink path a genuine
+/// failed load takes in production (never a crash, never a placeholder).
+void main() {
+  setUp(() {
+    SharedPreferences.setMockInitialValues({});
+  });
+
+  Widget construir({required bool premium}) {
+    return MultiProvider(
+      providers: [
+        ChangeNotifierProvider<EstadoEntitlement>(
+          create: (_) {
+            final estado = EstadoEntitlement(prefs: null);
+            return estado;
+          },
+        ),
+        Provider<ServicioAnuncios>(
+          create: (_) => ServicioAnuncios(esPremium: () => premium),
+        ),
+      ],
+      child: MaterialApp(
+        home: Scaffold(
+          body: Column(
+            children: [
+              const BannerAnuncioSuperior(),
+              const Expanded(child: Center(child: Text('contenido'))),
+            ],
+          ),
+        ),
+      ),
+    );
+  }
+
+  testWidgets(
+    'usuario free sin anuncio cargado: colapsa a SizedBox.shrink (nunca overlay)',
+    (tester) async {
+      await tester.pumpWidget(construir(premium: false));
+      await tester.pump();
+      await tester.pump(const Duration(milliseconds: 50));
+
+      final banner = find.byType(BannerAnuncioSuperior);
+      expect(banner, findsOneWidget);
+      // Collapsed to zero footprint (no ad ever loads in a widget test — no
+      // AdMob plugin channel registered) — a `SizedBox.shrink()`, not an
+      // overlay: the Column layout below it stays fully visible.
+      expect(tester.getSize(banner).height, 0);
+      expect(find.text('contenido'), findsOneWidget);
+    },
+  );
+
+  testWidgets('usuario premium: nunca intenta mostrar el banner', (
+    tester,
+  ) async {
+    await tester.pumpWidget(construir(premium: true));
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 50));
+
+    final banner = find.byType(BannerAnuncioSuperior);
+    expect(tester.getSize(banner).height, 0);
+    expect(find.text('contenido'), findsOneWidget);
+  });
+}

test/widgets/pluri_push_scaffold_test.dart

@@ -5,6 +5,7 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:pluriwave/estado/estado_alarmas.dart';
 import 'package:pluriwave/estado/estado_busqueda.dart';
 import 'package:pluriwave/estado/estado_ecualizador.dart';
+import 'package:pluriwave/estado/estado_entitlement.dart';
 import 'package:pluriwave/estado/estado_grabacion.dart';
 import 'package:pluriwave/estado/estado_idioma.dart';
 import 'package:pluriwave/estado/estado_radio.dart';
@@ -213,6 +214,9 @@ void main() {
             ),
             ListenableProvider<EstadoGrabacion>.value(value: estado.grabacion),
             ChangeNotifierProvider<EstadoIdioma>.value(value: estadoIdioma),
+            ChangeNotifierProvider<EstadoEntitlement>(
+              create: (_) => EstadoEntitlement(prefs: null),
+            ),
           ],
           child: testApp(const PantallaAjustes()),
         ),