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/main.dart

@@ -4,8 +4,10 @@ import 'dart:ui' as ui;
 import 'package:audio_service/audio_service.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
+import 'package:shared_preferences/shared_preferences.dart';
 import 'app.dart';
 import 'servicios/servicio_audio.dart';
+import 'servicios/servicio_audio_session.dart';
 
 const _anchoMinimoLandscape = 600.0;
 
@@ -13,6 +15,10 @@ Future<void> main() async {
   WidgetsFlutterBinding.ensureInitialized();
   await _aplicarPoliticaOrientacion();
 
+  // S3-R4: single SharedPreferences instance resolved once at startup and
+  // injected into every state/service below.
+  final prefs = await SharedPreferences.getInstance();
+
   final handler = await AudioService.init(
     builder: () => PluriWaveAudioHandler(),
     config: const AudioServiceConfig(
@@ -25,7 +31,12 @@ Future<void> main() async {
   );
   registrarHandler(handler);
 
-  runApp(const _OrientacionResponsiveApp(child: PluriWaveApp()));
+  // S3-R1: audio focus — phone calls / transient losses pause or duck the
+  // radio; headphones unplugged pauses it.
+  final sesionAudio = ServicioAudioSession(objetivo: handler);
+  unawaited(sesionAudio.configurar());
+
+  runApp(_OrientacionResponsiveApp(child: PluriWaveApp(prefs: prefs)));
 }
 
 Future<void> _aplicarPoliticaOrientacion([ui.Display? display]) async {
@@ -36,12 +47,9 @@ Future<void> _aplicarPoliticaOrientacion([ui.Display? display]) async {
   final displayActivo = display ?? vista?.display;
   if (displayActivo == null) return;
 
-  final anchoLogico =
-      displayActivo.size.width / displayActivo.devicePixelRatio;
+  final anchoLogico = displayActivo.size.width / displayActivo.devicePixelRatio;
   if (anchoLogico < _anchoMinimoLandscape) {
-    await SystemChrome.setPreferredOrientations([
-      DeviceOrientation.portraitUp,
-    ]);
+    await SystemChrome.setPreferredOrientations([DeviceOrientation.portraitUp]);
     return;
   }