refactor(state): extract recording and search state, scope screen rebuilds

- New EstadoGrabacion owns the recording service, subscription, directory/size preferences and open-file actions
- New EstadoBusqueda owns search, nearby stations, pagination and the min-bitrate filter
- New orden_emisoras.dart with the OrdenEmisoras enum, shared sorter and list identity memoization so context.select comparisons work on derived lists
- Large screens (inicio, buscar, favoritos, ajustes, reproductor) consume scoped selects/dedicated notifiers instead of root context.watch<EstadoRadio>, so audio buffer events no longer rebuild whole screens
- Remove all 15 TODO(S4b) compat members from EstadoRadio; consumers use the dedicated providers. EstadoRadio drops from ~1121 to 753 lines, keeping playback/stations/favorites orchestration
- 8 new tests including a rebuild-scoping probe (110 total green), flutter analyze clean

lib/app.dart

@@ -2,7 +2,9 @@ import 'dart:async';
 import 'package:flutter/material.dart';
 import 'package:provider/provider.dart';
 import 'package:shared_preferences/shared_preferences.dart';
+import 'estado/estado_busqueda.dart';
 import 'estado/estado_ecualizador.dart';
+import 'estado/estado_grabacion.dart';
 import 'estado/estado_radio.dart';
 import 'estado/estado_alarmas.dart';
 import 'estado/estado_idioma.dart';
@@ -36,12 +38,19 @@ class PluriWaveApp extends StatelessWidget {
     return MultiProvider(
       providers: [
         ChangeNotifierProvider(create: (_) => EstadoRadio(prefs: prefs)),
-        // EQ notifier (S4-R1). Created and disposed by EstadoRadio during
-        // the S4 transition; this provider only exposes the instance, so it
-        // declares no dispose callback.
+        // Domain notifiers (S4-R1/R2/R3). Created and disposed by EstadoRadio
+        // (they need its services and callbacks at construction); these
+        // providers only expose the instances, so they declare no dispose
+        // callback.
         ListenableProvider<EstadoEcualizador>(
           create: (context) => context.read<EstadoRadio>().ecualizador,
         ),
+        ListenableProvider<EstadoGrabacion>(
+          create: (context) => context.read<EstadoRadio>().grabacion,
+        ),
+        ListenableProvider<EstadoBusqueda>(
+          create: (context) => context.read<EstadoRadio>().busqueda,
+        ),
         ChangeNotifierProvider(create: (_) => EstadoAlarmas(prefs: prefs)),
         ChangeNotifierProvider(
           create: (_) => EstadoIdioma(sharedPreferences: prefs),