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_ecualizador.dart

@@ -17,12 +17,20 @@ class ConfiguracionEcualizador {
 }
 
 class ServicioEcualizador {
+  ServicioEcualizador({SharedPreferences? prefs}) : _prefs = prefs;
+
   static const _keyPresetPrincipal = 'eq_preset_principal_v1';
   static const _keyPresetsPorEmisora = 'eq_presets_por_emisora_v1';
   static const _keyActivo = 'eq_activo_v1';
 
+  final SharedPreferences? _prefs;
+
+  /// Injected startup instance (S3-R4); getInstance() is only a fallback.
+  Future<SharedPreferences> _resolverPrefs() async =>
+      _prefs ?? SharedPreferences.getInstance();
+
   Future<ConfiguracionEcualizador> cargar() async {
-    final prefs = await SharedPreferences.getInstance();
+    final prefs = await _resolverPrefs();
     final principal = _leerPresetPrincipal(prefs);
     final porEmisora = _leerPresetsPorEmisora(prefs);
     return ConfiguracionEcualizador(
@@ -33,31 +41,31 @@ class ServicioEcualizador {
   }
 
   Future<void> guardarPrincipal(PresetEcualizador preset) async {
-    final prefs = await SharedPreferences.getInstance();
+    final prefs = await _resolverPrefs();
     await prefs.setString(_keyPresetPrincipal, jsonEncode(preset.toJson()));
   }
 
   Future<void> guardarPorEmisora(String uuid, PresetEcualizador preset) async {
-    final prefs = await SharedPreferences.getInstance();
+    final prefs = await _resolverPrefs();
     final mapa = _leerPresetsPorEmisora(prefs);
     mapa[uuid] = preset;
     await _guardarPresetsPorEmisora(prefs, mapa);
   }
 
   Future<void> guardarActivo(bool activo) async {
-    final prefs = await SharedPreferences.getInstance();
+    final prefs = await _resolverPrefs();
     await prefs.setBool(_keyActivo, activo);
   }
 
   Future<void> eliminarPorEmisora(String uuid) async {
-    final prefs = await SharedPreferences.getInstance();
+    final prefs = await _resolverPrefs();
     final mapa = _leerPresetsPorEmisora(prefs);
     mapa.remove(uuid);
     await _guardarPresetsPorEmisora(prefs, mapa);
   }
 
   Future<void> guardarConfiguracion(ConfiguracionEcualizador config) async {
-    final prefs = await SharedPreferences.getInstance();
+    final prefs = await _resolverPrefs();
     await prefs.setString(
       _keyPresetPrincipal,
       jsonEncode(config.principal.toJson()),