feat(auto): browse and play local music folders in Android Auto [size:exception]

Phase 1: pick a device folder via SAF (persisted grant, no new
permission), browse its nested subfolders/tracks as a 5th Android
Auto root folder (hidden until configured), and play tracks through
the existing pipeline (EQ, art rotation, cold-start-safe source).
No metadata/sort/filter/shuffle yet -- filename is the title, generic
rotating art is the placeholder; deferred to a follow-up phase.

Adds a new pluriwave/file_actions native method (listAudioChildren)
and an onActivityResult override in MainActivity for the SAF folder
picker -- both static-review-only, no Android build available here.

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 'musica_local_auto.dart';
 import 'navegacion_auto.dart';
 import 'servicio_audio_session.dart';
 import 'servicio_ecualizador.dart';
@@ -47,6 +48,17 @@ void registrarFuenteNavegacion(FuenteEmisorasAuto fuente) {
   _fuenteNavegacionGlobal = fuente;
 }
 
+/// Local-music browse source — registered from main.dart, mirrors
+/// [registrarFuenteNavegacion] above (Design "getChildren data source
+/// registration"). `null` until registered (headless cold bind before
+/// main.dart's registration line runs) — every consumer below treats a
+/// `null` fuente as "not configured" rather than throwing.
+FuenteMusicaLocalAuto? _fuenteMusicaLocalGlobal;
+
+void registrarFuenteMusicaLocal(FuenteMusicaLocalAuto fuente) {
+  _fuenteMusicaLocalGlobal = fuente;
+}
+
 /// Wrapper de alto nivel para el UI.
 class ServicioAudio {
   PluriWaveAudioHandler get _handler {
@@ -735,12 +747,20 @@ class PluriWaveAudioHandler extends BaseAudioHandler
   ]) async {
     try {
       final constructor = ConstructorArbolAuto();
+      final fuenteLocal = _fuenteMusicaLocalGlobal;
       if (parentMediaId == AudioService.browsableRootId) {
-        return constructor.raiz();
+        final incluirMusicaLocal =
+            fuenteLocal != null && await fuenteLocal.hayCarpetaConfigurada();
+        return constructor.raiz(incluirMusicaLocal: incluirMusicaLocal);
       }
       if (parentMediaId == ConstructorArbolAuto.idEcualizador) {
         return constructor.presetsEq(PresetEcualizador.presets);
       }
+      final musicaLocal = await hijosMusicaLocal(
+        parentMediaId,
+        fuente: fuenteLocal,
+      );
+      if (musicaLocal != null) return musicaLocal;
       final fuente = _fuenteNavegacionGlobal;
       if (fuente == null) return const [];
       if (parentMediaId == ConstructorArbolAuto.idFavoritos) {
@@ -802,6 +822,20 @@ class PluriWaveAudioHandler extends BaseAudioHandler
         );
         return;
       }
+      // Local-track playback (Design "Local Track Playback Reuses Existing
+      // Pipeline", Spec "User selects a local track"): SECOND branch,
+      // unconditional `return`, mirroring the eq_preset branch above — a
+      // `pista:` id never falls through to the station routing below.
+      if (esPistaMediaId(mediaId)) {
+        final fuenteLocal = _fuenteMusicaLocalGlobal;
+        if (fuenteLocal == null) return;
+        await reproducirPistaLocal(
+          mediaId,
+          fuente: fuenteLocal,
+          reproducir: playMediaItem,
+        );
+        return;
+      }
       final fuente = _fuenteNavegacionGlobal;
       if (fuente == null) return;
       await reproducirPorMediaId(