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

test/helpers/fakes_alarmas.dart

@@ -1,5 +1,6 @@
 import 'dart:async';
 
+import 'package:pluriwave/l10n/gen/app_localizations.dart';
 import 'package:pluriwave/modelos/alarma_musical.dart';
 import 'package:pluriwave/servicios/servicio_alarmas_android.dart';
 import 'package:pluriwave/servicios/servicio_grabacion_radio.dart';
@@ -22,6 +23,9 @@ class FakePuertoAlarmasAndroid implements PuertoAlarmasAndroid {
   @override
   Stream<EventoAlarmaAndroid> get eventosAlarma => _eventos.stream;
 
+  @override
+  void configurarLocalizaciones(AppLocalizations l10n) {}
+
   @override
   Future<void> programar(AlarmaMusical alarma) async {
     programadas.add(alarma);