feat(auto): browsable Android Auto media tree with play-by-id [size:exception]

Expose PluriWave to Android Auto (projected) as a media app:

- Declare car media support (automotive_app_desc.xml + manifest meta-data)
  so Android Auto discovers the existing MediaBrowserService.
- New navegacion_auto.dart: ConstructorArbolAuto builds the browse tree
  (Favoritos / Todas las emisoras / Mis emisoras, 50-item cap, stable
  emisora:<id> media ids), reproducirPorMediaId routes a car tap to the
  existing playMediaItem pipeline, FuenteEmisorasAutoLocal serves the tree
  cold-start-safe (local favorites/custom stations before Flutter UI runs).
- PluriWaveAudioHandler overrides getChildren/getMediaItem/playFromMediaId
  as thin delegations; playback pipeline untouched.
- EstadoRadio pushes live station snapshots to the browse source and
  reconciles the selected station when playback starts from the car.
- Every playable item ships title + artUri; stations without logo fall
  back to a bundled default art (android.resource://).

Tests: 52/52 green (10 new navegacion_auto, 2 new estado_radio, plus
audio safety-net suites). Handler overrides and native XML are
static-review-only (no Android build env). Size exception approved for a
single reviewable commit.

lib/servicios/servicio_audio.dart

@@ -11,6 +11,7 @@ import '../l10n/gen/app_localizations.dart';
 import '../modelos/emisora.dart';
 import '../modelos/preset_ecualizador.dart';
 import 'controlador_reconexion.dart';
+import 'navegacion_auto.dart';
 import 'servicio_audio_session.dart';
 
 /// Estado de reproducción expuesto al UI.
@@ -35,6 +36,16 @@ void registrarHandler(PluriWaveAudioHandler handler) {
   _handlerGlobal = handler;
 }
 
+// ─────────────────────────────────────────────────────────────────────────────
+// Android Auto browse source — registered from main.dart, mirrors
+// registrarHandler above (Design "getChildren data source registration").
+// ─────────────────────────────────────────────────────────────────────────────
+FuenteEmisorasAuto? _fuenteNavegacionGlobal;
+
+void registrarFuenteNavegacion(FuenteEmisorasAuto fuente) {
+  _fuenteNavegacionGlobal = fuente;
+}
+
 /// Wrapper de alto nivel para el UI.
 class ServicioAudio {
   PluriWaveAudioHandler get _handler {
@@ -712,4 +723,84 @@ class PluriWaveAudioHandler extends BaseAudioHandler
       favicon: mediaItem.artUri?.toString(),
     );
   }
+
+  // ── Android Auto browsing (thin delegation to navegacion_auto.dart's
+  // already-tested pure logic — Design "getChildren data source") ─────────
+
+  @override
+  Future<List<MediaItem>> getChildren(
+    String parentMediaId, [
+    Map<String, dynamic>? options,
+  ]) async {
+    try {
+      final constructor = ConstructorArbolAuto();
+      if (parentMediaId == AudioService.browsableRootId) {
+        return constructor.raiz();
+      }
+      final fuente = _fuenteNavegacionGlobal;
+      if (fuente == null) return const [];
+      final emisoras = await _listaParaCarpeta(fuente, parentMediaId);
+      return constructor.hijos(parentMediaId, emisoras: emisoras);
+    } catch (_) {
+      // Spec "Browse requested before app state is loaded": never throw out
+      // of a browse call, even on an unexpected failure.
+      return const [];
+    }
+  }
+
+  @override
+  Future<MediaItem?> getMediaItem(String mediaId) async {
+    try {
+      final fuente = _fuenteNavegacionGlobal;
+      if (fuente == null) return null;
+      final universo = await _universoCompleto(fuente);
+      final constructor = ConstructorArbolAuto();
+      final emisora = constructor.resolver(mediaId, universo);
+      return emisora == null ? null : constructor.itemEmisora(emisora);
+    } catch (_) {
+      return null;
+    }
+  }
+
+  @override
+  Future<void> playFromMediaId(
+    String mediaId, [
+    Map<String, dynamic>? extras,
+  ]) async {
+    final fuente = _fuenteNavegacionGlobal;
+    if (fuente == null) return;
+    try {
+      await reproducirPorMediaId(
+        mediaId,
+        fuente: fuente,
+        reproducir: playMediaItem,
+      );
+    } catch (e) {
+      // Spec "Unknown or stale media id": never propagate from the handler.
+      developer.log(
+        '[PluriWave] Error en playFromMediaId($mediaId): $e',
+        name: 'ServicioAudio',
+        level: 900,
+      );
+    }
+  }
+
+  Future<List<Emisora>> _listaParaCarpeta(
+    FuenteEmisorasAuto fuente,
+    String parentId,
+  ) => switch (parentId) {
+    ConstructorArbolAuto.idFavoritos => fuente.favoritos(),
+    ConstructorArbolAuto.idMisEmisoras => fuente.misEmisoras(),
+    ConstructorArbolAuto.idTodas => fuente.todas(),
+    _ => Future.value(const []),
+  };
+
+  Future<List<Emisora>> _universoCompleto(FuenteEmisorasAuto fuente) async {
+    final listas = await Future.wait([
+      fuente.favoritos(),
+      fuente.misEmisoras(),
+      fuente.todas(),
+    ]);
+    return listas.expand((lista) => lista).toList();
+  }
 }