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/servicios/servicio_alarmas_android_instance_test.dart

@@ -0,0 +1,53 @@
+import 'package:flutter/services.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:pluriwave/servicios/servicio_alarmas_android.dart';
+
+/// S3-R2: the event controller and handler flag must be instance state so
+/// two bridges created independently never share events.
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+
+  Future<void> emitirAlarmFired(String canal, Map<String, Object?> payload) {
+    final mensaje = const StandardMethodCodec().encodeMethodCall(
+      MethodCall('alarmFired', payload),
+    );
+    return TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
+        .handlePlatformMessage(canal, mensaje, (_) {});
+  }
+
+  test('dos instancias no comparten el stream de eventos (S3-R2-A)', () async {
+    final servicioA = ServicioAlarmasAndroid(
+      channel: const MethodChannel('pluriwave/alarm_scheduler_test_a'),
+    );
+    final servicioB = ServicioAlarmasAndroid(
+      channel: const MethodChannel('pluriwave/alarm_scheduler_test_b'),
+    );
+
+    final eventosA = <EventoAlarmaAndroid>[];
+    final eventosB = <EventoAlarmaAndroid>[];
+    final subA = servicioA.eventosAlarma.listen(eventosA.add);
+    final subB = servicioB.eventosAlarma.listen(eventosB.add);
+    addTearDown(subA.cancel);
+    addTearDown(subB.cancel);
+
+    await emitirAlarmFired('pluriwave/alarm_scheduler_test_a', {
+      'alarmId': 'solo-a',
+      'alarmTitle': 'Alarma A',
+      'alarmAction': 'es.freetimelab.pluriwave.ALARM_FIRE',
+    });
+    await Future<void>.delayed(Duration.zero);
+
+    expect(eventosA.map((e) => e.alarmaId), ['solo-a']);
+    expect(eventosB, isEmpty, reason: 'B no debe ver los eventos de A');
+
+    await emitirAlarmFired('pluriwave/alarm_scheduler_test_b', {
+      'alarmId': 'solo-b',
+      'alarmTitle': 'Alarma B',
+      'alarmAction': 'es.freetimelab.pluriwave.ALARM_FIRE',
+    });
+    await Future<void>.delayed(Duration.zero);
+
+    expect(eventosA.map((e) => e.alarmaId), ['solo-a']);
+    expect(eventosB.map((e) => e.alarmaId), ['solo-b']);
+  });
+}