feat(audio): log AudioService.asyncError instead of swallowing it

`AudioService.asyncError` had ZERO subscribers app-wide. The plugin funnels
every asynchronous failure of its own observers into that stream and nowhere
else — `_observePlaybackState`, `_observeMediaItem` and `_observeQueue` each
wrap their whole body in `catch (e) { _asyncError.add(e); }`, and the artwork
path uses `.catchError(_asyncError.add)` — and a `PublishSubject` with no
listeners simply drops what it is given. The platform-side exception behind
"the media playback notification disappeared" was therefore being discarded
without a single log line, which is why that report arrives with no evidence
attached.

`observarErroresAudio` is a pure, injectable seam in `arranque_audio.dart`
(stream in, logger callback out), matching the seam convention this codebase
already uses for `esperarArranqueAudio`, `decidirAvanceCola` and
`debeReaplicarEcualizador`: the unit tests exercise the wiring with a plain
`StreamController`, never the real plugin. The default logger emits one
`[PluriWave]`-prefixed `developer.log` line at `level: 900`, the same level
and prefix `servicio_audio.dart` already uses, so one logcat filter catches
both.

Wired from `lib/main.dart`, not from `arranque_audio.dart`: main.dart is the
module that genuinely owns handler lifecycle — it is the only caller of
`AudioService.init`, `registrarHandler` and `ServicioAudioSession`, and both
the on-time and the degraded/timeout startup branches converge on its
`conectarHandler` closure. `arranque_audio.dart` owns only the timeout race
and the degraded loading shell; it never creates or registers a handler
(`alListo` is injected into it from main.dart), so it has no lifecycle to
hang a subscription on. Subscribing happens before `AudioService.init` — the
getter only touches a static subject — so nothing reported during the
MediaBrowser handshake is missed, and one subscription covers both paths.

The subscription is cancellable and its `cancel` is registered into the
handler via `registrarLimpiezaArranque`, mirroring the existing
`registrarHandler` / `registrarFuenteNavegacion` / `registrarFuenteMusicaLocal`
registration convention. `onTaskRemoved` — the only handler teardown in this
app — runs it, so the subscription cannot outlive what it instruments. The
dependency points bootstrap -> service, so `servicio_audio.dart` never has to
import the bootstrap module or the plugin's static stream.

Zero behaviour change: nothing but log output is added.

test/servicios/arranque_audio_test.dart

@@ -63,4 +63,93 @@ void main() {
       expect(handler, 'handler-tardio');
     });
   });
+
+  /// fix/notificacion-media — commit 1: `AudioService.asyncError` had zero
+  /// subscribers, so every exception `audio_service` swallows internally was
+  /// dropped on the floor. These cover the injectable seam only (Design
+  /// "Testability" — the stream and the logger are both injected), never the
+  /// real plugin.
+  group('observarErroresAudio', () {
+    test('reenvia al logger cada error emitido, en orden', () async {
+      final controlador = StreamController<Object>.broadcast();
+      final registrados = <Object>[];
+
+      final sub = observarErroresAudio(
+        controlador.stream,
+        registrar: registrados.add,
+      );
+
+      controlador.add('fallo-1');
+      controlador.add(StateError('fallo-2'));
+      await controlador.close();
+
+      expect(registrados, hasLength(2));
+      expect(registrados.first, 'fallo-1');
+      expect(registrados.last, isA<StateError>());
+
+      await sub.cancel();
+    });
+
+    test('cancelar la suscripcion corta el logging — no puede filtrarse '
+        'tras el teardown del handler', () async {
+      final controlador = StreamController<Object>.broadcast();
+      final registrados = <Object>[];
+
+      final sub = observarErroresAudio(
+        controlador.stream,
+        registrar: registrados.add,
+      );
+
+      controlador.add('antes-del-cancel');
+      // Deja que el evento se entregue antes de cancelar (los broadcast
+      // controllers entregan en un microtask, no de forma sincrona).
+      await Future<void>.delayed(Duration.zero);
+      await sub.cancel();
+
+      controlador.add('despues-del-cancel');
+      await controlador.close();
+
+      expect(
+        registrados,
+        ['antes-del-cancel'],
+        reason:
+            'tras cancelar, la suscripcion no debe seguir viva ni registrar '
+            'nada mas',
+      );
+    });
+
+    test(
+      'un evento de error del propio stream tambien llega al logger',
+      () async {
+        final controlador = StreamController<Object>.broadcast();
+        final registrados = <Object>[];
+
+        final sub = observarErroresAudio(
+          controlador.stream,
+          registrar: registrados.add,
+        );
+
+        // Rama defensiva: el plugin solo usa `add`, nunca `addError`, pero un
+        // error de stream sin manejar seria una excepcion no capturada.
+        controlador.addError(const FormatException('stream roto'));
+        await controlador.close();
+
+        expect(registrados, hasLength(1));
+        expect(registrados.single, isA<FormatException>());
+
+        await sub.cancel();
+      },
+    );
+
+    test('el logger por defecto acepta cualquier objeto sin lanzar', () {
+      expect(
+        () => registrarErrorAudioService(StateError('cualquier cosa')),
+        returnsNormally,
+      );
+      expect(
+        () => registrarErrorAudioService('un string suelto'),
+        returnsNormally,
+      );
+    });
+  });
 }