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.

lib/servicios/navegacion_auto.dart

@@ -333,13 +333,43 @@ class ConstructorArbolAuto {
   /// [incluirMusicaLocal] is `true` (Design "Local root hidden until a folder
   /// is configured") — the caller passes `fuente.hayCarpetaConfigurada()`,
   /// keeping this builder itself synchronous and side-effect free.
-  List<MediaItem> raiz({required bool incluirMusicaLocal}) => [
+  ///
+  /// [premium] (iap-freemium-unlock, Design ADR-4): the ROOT keeps the exact
+  /// same visible folder labels for every tier — "keeps the same visible
+  /// folder labels for free users" is the explicit design choice, so a free
+  /// driver still sees a real, familiar menu rather than a wall of "Función
+  /// Premium" rows. The lock itself is enforced one level DOWN, at the
+  /// `getChildren` choke point (see [itemPremiumBloqueado] and
+  /// [respuestaBloqueadaPorEntitlement] below) — tapping any of these
+  /// folders as a free user reveals the lock there, never here.
+  List<MediaItem> raiz({
+    required bool incluirMusicaLocal,
+    required bool premium,
+  }) => [
     _carpeta(idFavoritos, 'Favoritos'),
     _carpeta(idTodas, 'Todas las emisoras'),
     _carpeta(idMisEmisoras, 'Mis emisoras'),
     if (incluirMusicaLocal) _carpeta(idMusicaLocal, 'Música Local'),
   ];
 
+  /// Free-tier id prefix reserved id (iap-freemium-unlock, Design ADR-4):
+  /// the single non-playable item every non-root folder collapses to for a
+  /// free-tier user. Hardcoded Spanish label, matching every other car-tree
+  /// label in this file (never routed through `AppLocalizations` —
+  /// established convention, see [_tituloMasLocal]'s doc).
+  static const idPremiumInfo = 'premium:info';
+
+  /// The single locked item shown for ANY non-root folder when the browsing
+  /// user is free tier (Design ADR-4, android-auto-media spec "Free-Tier
+  /// Reduced Root Browse"). Non-playable — selecting it is a no-op, never a
+  /// crash (Spec "Free-tier user selects a locked item").
+  MediaItem itemPremiumBloqueado() => MediaItem(
+    id: idPremiumInfo,
+    title: 'Función Premium',
+    playable: false,
+    extras: _contentStyleLista,
+  );
+
   MediaItem _carpeta(String id, String titulo) => MediaItem(
     id: id,
     title: titulo,
@@ -899,6 +929,26 @@ class ConstructorArbolAuto {
   }
 }
 
+/// Pure Android Auto browse-gate decision (iap-freemium-unlock, Design
+/// ADR-4): the AUTHORITATIVE `getChildren` choke point, called BEFORE any
+/// other resolution. For the root itself this NEVER blocks (the root always
+/// resolves through [ConstructorArbolAuto.raiz] instead, which stays
+/// visible for every tier). For any non-root [parentMediaId] and a free-tier
+/// [premium], it returns the single locked item regardless of what the id
+/// actually is — a stale/deep-linked `emisora:<uuid>` or folder id from
+/// before a downgrade is blocked exactly the same way as a legitimate
+/// current folder id (android-auto-media spec "Free-Tier Browse Never
+/// Leaks Real Content (Authoritative Backstop)"). Returns `null` when the
+/// caller should proceed with its normal resolution (root, or premium).
+List<MediaItem>? respuestaBloqueadaPorEntitlement({
+  required String parentMediaId,
+  required bool premium,
+}) {
+  if (parentMediaId == AudioService.browsableRootId) return null;
+  if (premium) return null;
+  return [ConstructorArbolAuto().itemPremiumBloqueado()];
+}
+
 /// Routing seam between a car-tapped `emisora:<uuid>` media id and the
 /// existing internal playback path (Design "playback coherence" — reuse
 /// over duplication). Resolves the uuid via [fuente], builds the same