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/estado/estado_grabacion_gating_test.dart

@@ -0,0 +1,95 @@
+import 'dart:async';
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:pluriwave/estado/estado_grabacion.dart';
+import 'package:pluriwave/modelos/emisora.dart';
+import 'package:pluriwave/servicios/servicio_grabacion_radio.dart';
+
+import '../helpers/fakes.dart';
+
+/// Freemium gating (freemium-gating spec "Recording Start Gated, Management
+/// Stays Free"): starting a NEW recording requires premium; management of
+/// already-existing recordings (listing/playing/deleting — untouched by
+/// this file) stays free regardless.
+void main() {
+  test(
+    'free tier: iniciar() no llama al servicio y reporta requierePremium',
+    () async {
+      final servicio = _ServicioGrabacionControlado();
+      final emisora = emisoraDemo(uuid: 'rec-1', nombre: 'Grabable');
+      final estado = EstadoGrabacion(
+        servicio: servicio,
+        emisoraActual: () => emisora,
+        esPremium: () => false,
+      );
+      addTearDown(estado.dispose);
+
+      final resultado = await estado.iniciar();
+
+      expect(resultado, ResultadoIniciarGrabacion.requierePremium);
+      expect(servicio.inicios, 0);
+    },
+  );
+
+  test('premium: iniciar() delega en el servicio normalmente', () async {
+    final servicio = _ServicioGrabacionControlado();
+    final emisora = emisoraDemo(uuid: 'rec-2', nombre: 'Grabable');
+    final estado = EstadoGrabacion(
+      servicio: servicio,
+      emisoraActual: () => emisora,
+      esPremium: () => true,
+    );
+    addTearDown(estado.dispose);
+
+    final resultado = await estado.iniciar(
+      duracion: const Duration(minutes: 1),
+    );
+
+    expect(resultado, ResultadoIniciarGrabacion.iniciada);
+    expect(servicio.inicios, 1);
+  });
+
+  test('sin callback de entitlement, el default no bloquea (compat)', () async {
+    final servicio = _ServicioGrabacionControlado();
+    final emisora = emisoraDemo(uuid: 'rec-3', nombre: 'Grabable');
+    final estado = EstadoGrabacion(
+      servicio: servicio,
+      emisoraActual: () => emisora,
+    );
+    addTearDown(estado.dispose);
+
+    final resultado = await estado.iniciar();
+
+    expect(resultado, ResultadoIniciarGrabacion.iniciada);
+    expect(servicio.inicios, 1);
+  });
+}
+
+class _ServicioGrabacionControlado extends ServicioGrabacionRadio {
+  final _controller = StreamController<EstadoGrabacionRadio>.broadcast();
+  final EstadoGrabacionRadio _estadoActual =
+      const EstadoGrabacionRadio.inactiva();
+
+  int inicios = 0;
+
+  @override
+  EstadoGrabacionRadio get estado => _estadoActual;
+
+  @override
+  Stream<EstadoGrabacionRadio> get estadoStream => _controller.stream;
+
+  @override
+  Future<void> inicializar() async {}
+
+  @override
+  Future<void> iniciar(
+    Emisora emisora, {
+    Duration? duracion,
+    String? directorio,
+  }) async {
+    inicios++;
+  }
+
+  @override
+  Future<void> dispose() => _controller.close();
+}