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/main.dart

@@ -6,6 +6,7 @@ import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 import 'app.dart';
+import 'servicios/navegacion_auto.dart';
 import 'servicios/servicio_audio.dart';
 import 'servicios/servicio_audio_session.dart';
 import 'tema/pluriwave_tokens.dart';
@@ -41,12 +42,23 @@ Future<void> main() async {
   );
   registrarHandler(handler);
 
+  // Android Auto browse source (Design "getChildren data source, cold-start
+  // safe") — registered before EstadoRadio builds so a headless Auto bind
+  // (main() runs but the lazily-created Provider tree may never build) can
+  // still serve favourites/custom stations from local reads.
+  final fuenteAuto = FuenteEmisorasAutoLocal();
+  registrarFuenteNavegacion(fuenteAuto);
+
   // S3-R1: audio focus — phone calls / transient losses pause or duck the
   // radio; headphones unplugged pauses it.
   final sesionAudio = ServicioAudioSession(objetivo: handler);
   unawaited(sesionAudio.configurar());
 
-  runApp(_OrientacionResponsiveApp(child: PluriWaveApp(prefs: prefs)));
+  runApp(
+    _OrientacionResponsiveApp(
+      child: PluriWaveApp(prefs: prefs, fuenteAuto: fuenteAuto),
+    ),
+  );
 }
 
 Future<void> _aplicarPoliticaOrientacion([ui.Display? display]) async {