feat(audio): audio session integration and runtime robustness

- Integrate audio_session (new servicio_audio_session.dart): incoming calls pause the radio and resume on end, headphone unplug pauses without auto-resume, permanent focus loss never auto-resumes, duck lowers volume
- Add play-intent flag to ServicioAudio so interruption handling and future reconnect logic can distinguish user pause from system-driven stops
- Eliminate read-modify-write race in ServicioAlarmas with an in-memory cache and single-writer queue across all mutations; recalcularTodas persists only when state actually changed
- Convert ServicioAlarmasAndroid static StreamController/handler to injectable instance fields, restoring test isolation
- Inject a single cached SharedPreferences from main.dart across services and state (removes 23 inline getInstance() calls)
- Move configurarLocalizaciones out of MiniReproductor.build() (was running on every rebuild during playback)
- Bound the alarm fire-dedup set (cap 200 entries, 24h pruning)
- 12 new tests (89 total green), flutter analyze clean

lib/servicios/servicio_audio.dart

@@ -9,6 +9,7 @@ import '../l10n/display_names.dart';
 import '../l10n/gen/app_localizations.dart';
 import '../modelos/emisora.dart';
 import '../modelos/preset_ecualizador.dart';
+import 'servicio_audio_session.dart';
 
 /// Estado de reproducción expuesto al UI.
 enum EstadoReproduccion { detenido, cargando, reproduciendo, pausado, error }
@@ -110,9 +111,12 @@ class ServicioAudio {
 // ─────────────────────────────────────────────────────────────────────────────
 // AudioHandler
 // ─────────────────────────────────────────────────────────────────────────────
-class PluriWaveAudioHandler extends BaseAudioHandler with SeekHandler {
+class PluriWaveAudioHandler extends BaseAudioHandler
+    with SeekHandler
+    implements ObjetivoAudioInterrumpible {
   static const _timeoutCambioFuente = Duration(seconds: 12);
   static const _timeoutCierrePlayer = Duration(seconds: 3);
+  static const _factorAtenuacion = 0.3;
 
   AndroidEqualizer _eq = AndroidEqualizer();
   late AudioPlayer _player = _crearPlayer();
@@ -130,6 +134,15 @@ class PluriWaveAudioHandler extends BaseAudioHandler with SeekHandler {
   double get volumen => _volumen;
   AppLocalizations? _l10n;
 
+  /// Intent-to-play flag (Designs 3.1/7.2): reflects the LAST explicit
+  /// intent (play/pause/stop, including audio-session interruptions, which
+  /// pause through [pausar]). The S7 reconnect state machine reads it to
+  /// distinguish a network stall from an intentional pause.
+  bool _intencionReproducir = false;
+
+  /// Ducked state requested by the audio session (transient focus loss).
+  bool _atenuado = false;
+
   AndroidEqualizer? get ecualizador => _eq;
   bool _eqDisponible = false;
   bool get ecualizadorDisponible => _eqDisponible;
@@ -278,6 +291,7 @@ class PluriWaveAudioHandler extends BaseAudioHandler with SeekHandler {
 
   @override
   Future<void> playMediaItem(MediaItem mediaItem) async {
+    _intencionReproducir = true;
     final revision = ++_revisionFuente;
     _colaCambioFuente = _colaCambioFuente
         .catchError((_) {})
@@ -349,7 +363,7 @@ class PluriWaveAudioHandler extends BaseAudioHandler with SeekHandler {
     _eqDisponible = false;
     _androidAudioSessionId = null;
     _player = _crearPlayer();
-    await _player.setVolume(_volumen);
+    await _player.setVolume(_volumenEfectivo);
     _conectarStreamsPlayer();
   }
 
@@ -450,17 +464,48 @@ class PluriWaveAudioHandler extends BaseAudioHandler with SeekHandler {
 
   Future<void> setVolumen(double vol) async {
     _volumen = vol.clamp(0.0, 1.0);
-    await _player.setVolume(_volumen);
+    await _player.setVolume(_volumenEfectivo);
+  }
+
+  double get _volumenEfectivo =>
+      _atenuado ? _volumen * _factorAtenuacion : _volumen;
+
+  // ── ObjetivoAudioInterrumpible (audio-session seam, S3-R1) ───────────────
+
+  @override
+  bool get intencionReproducir => _intencionReproducir;
+
+  @override
+  bool get estaReproduciendo => playbackState.value.playing;
+
+  @override
+  Future<void> pausar() => pause();
+
+  @override
+  Future<void> reanudar() => play();
+
+  @override
+  Future<void> setAtenuado(bool atenuado) async {
+    if (_atenuado == atenuado) return;
+    _atenuado = atenuado;
+    await _player.setVolume(_volumenEfectivo);
   }
 
   @override
-  Future<void> play() => _player.play();
+  Future<void> play() {
+    _intencionReproducir = true;
+    return _player.play();
+  }
 
   @override
-  Future<void> pause() => _player.pause();
+  Future<void> pause() {
+    _intencionReproducir = false;
+    return _player.pause();
+  }
 
   @override
   Future<void> stop() async {
+    _intencionReproducir = false;
     _revisionFuente++;
     await _player.stop();
     emisoraActual = null;