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/pantallas/pantalla_favoritos.dart

@@ -19,9 +19,15 @@ class PantallaFavoritos extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    final estado = context.watch<EstadoRadio>();
-    final favoritos = estado.listaFavoritos;
-    final grupos = estado.gruposFavoritos;
+    // S4-R5: no root watch — select only the fields this screen reads. The
+    // getters are identity-memoized, so playback notifications that do not
+    // change favorites/groups no longer rebuild the screen.
+    final favoritos = context.select<EstadoRadio, List<Emisora>>(
+      (e) => e.listaFavoritos,
+    );
+    final grupos = context.select<EstadoRadio, List<GrupoFavoritos>>(
+      (e) => e.gruposFavoritos,
+    );
     final l10n = AppLocalizations.of(context);
 
     if (favoritos.isEmpty) {
@@ -49,16 +55,17 @@ class PantallaFavoritos extends StatelessWidget {
       );
     }
 
-    final gruposVisibles = grupos.isEmpty
-        ? [
-            GrupoFavoritos(
-              id: GrupoFavoritos.sinAsignarId,
-              nombre: l10n.favoriteGroupsUnassigned,
-              orden: 0,
-              protegido: true,
-            ),
-          ]
-        : grupos;
+    final gruposVisibles =
+        grupos.isEmpty
+            ? [
+              GrupoFavoritos(
+                id: GrupoFavoritos.sinAsignarId,
+                nombre: l10n.favoriteGroupsUnassigned,
+                orden: 0,
+                protegido: true,
+              ),
+            ]
+            : grupos;
 
     return CustomScrollView(
       slivers: [
@@ -86,9 +93,10 @@ class PantallaFavoritos extends StatelessWidget {
                 _GrupoFavoritosPanel(
                   grupo: grupo,
                   grupos: gruposVisibles,
-                  emisoras: favoritos
-                      .where((e) => e.grupoFavoritosId == grupo.id)
-                      .toList(),
+                  emisoras:
+                      favoritos
+                          .where((e) => e.grupoFavoritosId == grupo.id)
+                          .toList(),
                 ),
                 const SizedBox(height: 12),
               ],
@@ -125,7 +133,9 @@ class _GrupoFavoritosPanel extends StatelessWidget {
         children: [
           Row(
             children: [
-              Icon(grupo.esSinAsignar ? Icons.lock_rounded : Icons.folder_rounded),
+              Icon(
+                grupo.esSinAsignar ? Icons.lock_rounded : Icons.folder_rounded,
+              ),
               const SizedBox(width: 8),
               Expanded(
                 child: Text(
@@ -181,30 +191,31 @@ class _FavoritoItem extends StatelessWidget {
     final seleccionado = await showModalBottomSheet<String>(
       context: context,
       showDragHandle: true,
-      builder: (ctx) => SafeArea(
-        child: ListView(
-          shrinkWrap: true,
-          children: [
-            Padding(
-              padding: const EdgeInsets.fromLTRB(20, 4, 20, 12),
-              child: Text(
-                l10n.favoriteGroupsAssign,
-                style: Theme.of(ctx).textTheme.titleLarge,
-              ),
-            ),
-            for (final grupo in grupos)
-              ListTile(
-                leading: Icon(
-                  grupo.id == emisora.grupoFavoritosId
-                      ? Icons.radio_button_checked_rounded
-                      : Icons.radio_button_off_rounded,
+      builder:
+          (ctx) => SafeArea(
+            child: ListView(
+              shrinkWrap: true,
+              children: [
+                Padding(
+                  padding: const EdgeInsets.fromLTRB(20, 4, 20, 12),
+                  child: Text(
+                    l10n.favoriteGroupsAssign,
+                    style: Theme.of(ctx).textTheme.titleLarge,
+                  ),
                 ),
-                title: Text(_nombreVisible(l10n, grupo)),
-                onTap: () => Navigator.pop(ctx, grupo.id),
-              ),
-          ],
-        ),
-      ),
+                for (final grupo in grupos)
+                  ListTile(
+                    leading: Icon(
+                      grupo.id == emisora.grupoFavoritosId
+                          ? Icons.radio_button_checked_rounded
+                          : Icons.radio_button_off_rounded,
+                    ),
+                    title: Text(_nombreVisible(l10n, grupo)),
+                    onTap: () => Navigator.pop(ctx, grupo.id),
+                  ),
+              ],
+            ),
+          ),
     );
     if (seleccionado == null || !context.mounted) return;
     await context.read<EstadoRadio>().asignarGrupoFavorito(