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_grabacion_radio.dart

@@ -82,9 +82,11 @@ class ServicioGrabacionRadio {
     http.Client? cliente,
     Future<Directory> Function()? resolverDirectorioBase,
     DateTime Function()? reloj,
+    SharedPreferences? prefs,
   }) : _clienteExterno = cliente,
        _resolverDirectorioBase = resolverDirectorioBase,
-       _reloj = reloj ?? DateTime.now;
+       _reloj = reloj ?? DateTime.now,
+       _prefs = prefs;
 
   static const _claveDirectorio = 'grabacion_radio_directorio';
   static const _claveMaxBytes = 'grabacion_radio_max_bytes_v1';
@@ -93,6 +95,11 @@ class ServicioGrabacionRadio {
   final http.Client? _clienteExterno;
   final Future<Directory> Function()? _resolverDirectorioBase;
   final DateTime Function() _reloj;
+  final SharedPreferences? _prefs;
+
+  /// Injected startup instance (S3-R4); getInstance() is only a fallback.
+  Future<SharedPreferences> _resolverPrefs() async =>
+      _prefs ?? SharedPreferences.getInstance();
   final _estadoController = StreamController<EstadoGrabacionRadio>.broadcast();
   AppLocalizations? _l10n;
 
@@ -123,7 +130,7 @@ class ServicioGrabacionRadio {
 
   Future<void> inicializar() async {
     try {
-      final prefs = await SharedPreferences.getInstance();
+      final prefs = await _resolverPrefs();
       _directorioConfigurado = prefs.getString(_claveDirectorio);
       _maxBytes = prefs.getInt(_claveMaxBytes) ?? maxBytesPorDefecto;
     } catch (_) {
@@ -151,7 +158,7 @@ class ServicioGrabacionRadio {
     }
     _directorioConfigurado = normalizado;
     try {
-      final prefs = await SharedPreferences.getInstance();
+      final prefs = await _resolverPrefs();
       await prefs.setString(_claveDirectorio, normalizado);
     } catch (_) {}
     _emitir(_estado);
@@ -160,7 +167,7 @@ class ServicioGrabacionRadio {
   Future<void> limpiarDirectorioConfigurado() async {
     _directorioConfigurado = null;
     try {
-      final prefs = await SharedPreferences.getInstance();
+      final prefs = await _resolverPrefs();
       await prefs.remove(_claveDirectorio);
     } catch (_) {}
     _emitir(_estado);
@@ -172,7 +179,7 @@ class ServicioGrabacionRadio {
     }
     _maxBytes = bytes;
     try {
-      final prefs = await SharedPreferences.getInstance();
+      final prefs = await _resolverPrefs();
       await prefs.setInt(_claveMaxBytes, bytes);
     } catch (_) {}
     _emitir(_estado);