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_alarmas_android.dart

@@ -126,6 +126,10 @@ class EjecucionAlarmaNativa {
 abstract class PuertoAlarmasAndroid {
   Stream<EventoAlarmaAndroid> get eventosAlarma;
 
+  /// Provides the UI localizations used to localize the alarm/station names
+  /// sent to the native scheduler.
+  void configurarLocalizaciones(AppLocalizations l10n);
+
   Future<void> programar(AlarmaMusical alarma);
   Future<void> cancelar(String alarmaId);
   Future<void> ocultarNotificacionAlarma(String alarmaId);
@@ -145,22 +149,24 @@ class ServicioAlarmasAndroid implements PuertoAlarmasAndroid {
   ServicioAlarmasAndroid({
     MethodChannel channel = const MethodChannel('pluriwave/alarm_scheduler'),
   }) : _channel = channel {
-    _instalarHandler(_channel);
+    _instalarHandler();
   }
 
   final MethodChannel _channel;
-  static final _eventosController =
-      StreamController<EventoAlarmaAndroid>.broadcast();
-  static bool _handlerInstalado = false;
-  static AppLocalizations? _l10n;
 
-  static AppLocalizations get _textos {
+  // Instance state (S3-R2): each bridge owns its controller and l10n so
+  // independent instances never share events through globals.
+  final _eventosController = StreamController<EventoAlarmaAndroid>.broadcast();
+  AppLocalizations? _l10n;
+
+  AppLocalizations get _textos {
     final actual = _l10n;
     if (actual != null) return actual;
     return lookupAppLocalizations(const Locale('es'));
   }
 
-  static void configurarLocalizaciones(AppLocalizations l10n) {
+  @override
+  void configurarLocalizaciones(AppLocalizations l10n) {
     _l10n = l10n;
   }
 
@@ -334,10 +340,11 @@ class ServicioAlarmasAndroid implements PuertoAlarmasAndroid {
     return _channel.invokeMethod<void>(method, args);
   }
 
-  static void _instalarHandler(MethodChannel channel) {
-    if (_handlerInstalado) return;
-    _handlerInstalado = true;
-    channel.setMethodCallHandler((call) async {
+  // Installed once per instance from the constructor. Creating a second
+  // instance over the SAME channel re-binds the platform handler to the
+  // newest instance (production has exactly one instance per channel).
+  void _instalarHandler() {
+    _channel.setMethodCallHandler((call) async {
       if (call.method != 'alarmFired') return;
       final args = call.arguments;
       if (args is Map) {