fix(audio): keep local skips local, and stop a failed station blanking Auto

Three reported suspicions. Two confirmed by reading, one not.

1. CONFIRMED, self-inflicted. Playing a song from the phone and pressing
NEXT jumped to a radio station.

3398d02 taught skipToNext/skipToPrevious to fall back to station skipping
when there is no local queue, so the car's buttons would not be dead for
radio. But queue-less does not mean radio: tapping ONE track goes through
reproducirPistaLocal, which never builds a queue -- only folder playback
sets _colaLocal. That is exactly why the report said "at least the first
time".

emisoraActual cannot tell them apart either: _cambiarFuente fills it in for
every source, so a local MP3 arrives as an Emisora whose url is its
content:// document URI. The media id's scheme is the real discriminator,
the same test that already keeps the recorder off local files. A local
track now skips nowhere, which is the correct behaviour for a single item.

2. CONFIRMED mechanism. A failed station made the app disappear from the
Android Auto pane.

The error path published STATE_ERROR and then cleared everything:
`emisoraActual = null; mediaItem.add(null)`. That leaves the session in an
error state with no metadata at all, and Auto drops a session with nothing
to show -- reported as "if a station fails it seems to crash, and going to
1/3 it fails".

Both are kept now. Nothing outside servicio_audio.dart consumes mediaItem
(verified), so the phone is unaffected, and the car gains two things: the
screen can still name the station that failed instead of going blank, and
previous/next stay usable, so a driver can skip out of a dead station
instead of being stranded -- _saltarEmisora needs emisoraActual to know
where it is in the list. The error state itself is unchanged.

3. NOT CONFIRMED. A local track occasionally jumping to another one mid-play.

An advance requires a genuine `completed` from just_audio, so either the
player reports the end early -- plausible for a content:// SAF source,
whose duration is not always exact -- or something else moved the track.
Reading the code cannot separate those, so nothing was changed on a guess.
The advance now logs the decision with the processing state, position and
duration that caused it, so the next occurrence arrives with its reason
attached.

Tests: 1192 -> 1195.

lib/servicios/servicio_audio.dart

@@ -925,9 +925,22 @@ class PluriWaveAudioHandler extends BaseAudioHandler
         errorMessage: mensaje,
       ),
     );
-    emisoraActual = null;
-    mediaItem.add(null);
-
+    // The failed item and station are KEPT, deliberately.
+    //
+    // This used to do `emisoraActual = null; mediaItem.add(null);`, which
+    // left the media session in STATE_ERROR with no metadata at all. Android
+    // Auto drops a session with nothing to show, which is what made PluriWave
+    // vanish from the car pane the moment a station failed -- reported as
+    // "if a station fails it seems to crash, and going to 1/3 it fails".
+    //
+    // Keeping them costs nothing on the phone (nothing outside this file
+    // consumes `mediaItem`, verified) and buys two things in the car: the
+    // screen can still name the station that failed instead of going blank,
+    // and previous/next stay usable, so the driver skips out of a dead
+    // station instead of being stranded -- `_saltarEmisora` needs
+    // `emisoraActual` to know where it is in the list.
+    //
+    // The error state itself is unchanged: STATE_ERROR with the message.
     _player.stop().catchError((_) {});
   }
 
@@ -1126,6 +1139,21 @@ class PluriWaveAudioHandler extends BaseAudioHandler
       avanzandoCola: _avanzandoCola,
       trackCompletado: proc == ProcessingState.completed,
     );
+    // Reported: a local track sometimes jumped to another one on its own,
+    // without reaching the end and without anyone pressing a thing. Reading
+    // the code cannot settle it -- an advance here requires a genuine
+    // `completed` from just_audio, so either the player reports the end
+    // early (plausible for a `content://` SAF source, whose duration is not
+    // always exact) or something else moved the track. Rather than guess,
+    // log the transition with the state that caused it, so the next report
+    // arrives with the reason attached instead of another hypothesis.
+    if (decision != DecisionAvanceCola.ninguna) {
+      debugPrint(
+        '[PluriWave][ServicioAudio] avance de cola decision=${decision.name} '
+        'proc=${proc.name} pos=${_player.position} dur=${_player.duration} '
+        'pista=${mediaItem.value?.title}',
+      );
+    }
     switch (decision) {
       case DecisionAvanceCola.ninguna:
         return;
@@ -1495,7 +1523,7 @@ class PluriWaveAudioHandler extends BaseAudioHandler
   Future<void> skipToNext() async {
     final cola = _colaLocal;
     if (cola == null) {
-      await _saltarEmisora(haciaAtras: false);
+      if (_reproduciendoRadio) await _saltarEmisora(haciaAtras: false);
       return;
     }
     final siguiente = cola.conSiguiente();
@@ -1516,7 +1544,7 @@ class PluriWaveAudioHandler extends BaseAudioHandler
   Future<void> skipToPrevious() async {
     final cola = _colaLocal;
     if (cola == null) {
-      await _saltarEmisora(haciaAtras: true);
+      if (_reproduciendoRadio) await _saltarEmisora(haciaAtras: true);
       return;
     }
     final anterior = cola.conAnterior();
@@ -1524,6 +1552,26 @@ class PluriWaveAudioHandler extends BaseAudioHandler
     await _reproducirEntradaCola(anterior.actual);
   }
 
+  /// Whether what is playing right now is a RADIO STREAM, as opposed to a
+  /// local file.
+  ///
+  /// Reported: playing a single local track and pressing next jumped to a
+  /// radio station. `emisoraActual` cannot answer this — `_cambiarFuente`
+  /// fills it in for every source, so a local MP3 arrives as an `Emisora`
+  /// whose `url` is its `content://` document URI. The scheme of the media
+  /// id is what actually distinguishes them, and it is the same test
+  /// `esEmisoraGrabable` uses to keep the recorder off local files.
+  ///
+  /// Only a queue-less local track reaches this: folder playback sets
+  /// `_colaLocal` and skips within the queue, which is why the report said
+  /// "at least the first time" — tapping one track never builds a queue.
+  bool get _reproduciendoRadio {
+    final id = mediaItem.value?.id;
+    if (id == null) return false;
+    final esquema = Uri.tryParse(id)?.scheme.toLowerCase();
+    return esquema == 'http' || esquema == 'https';
+  }
+
   /// Station-to-station skipping for the car's transport row.
   ///
   /// The list to walk is resolved by [listaParaSaltoEmisora]: the narrowest