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/estado/orden_emisoras.dart

@@ -0,0 +1,55 @@
+import '../modelos/emisora.dart';
+
+/// User-selectable ordering for every station list in the app.
+enum OrdenEmisoras { nombre, calidad }
+
+/// Returns a sorted COPY of [emisoras] according to [orden].
+List<Emisora> ordenarEmisoras(List<Emisora> emisoras, OrdenEmisoras orden) {
+  final ordenadas = List<Emisora>.from(emisoras);
+  switch (orden) {
+    case OrdenEmisoras.nombre:
+      ordenadas.sort(
+        (a, b) => a.nombre.toLowerCase().compareTo(b.nombre.toLowerCase()),
+      );
+    case OrdenEmisoras.calidad:
+      ordenadas.sort((a, b) {
+        final porBitrate = (b.bitrate ?? 0).compareTo(a.bitrate ?? 0);
+        if (porBitrate != 0) return porBitrate;
+        return 0;
+      });
+  }
+  return ordenadas;
+}
+
+/// Identity-memoized derived list (S4-R5).
+///
+/// Derived-list getters used to return a fresh copy on every read, which made
+/// `context.select` rebuild on EVERY notification (lists compare by identity).
+/// This memo recomputes only when one of the source [claves] changes identity,
+/// so unrelated notifications (e.g. audio buffer events) stop rebuilding the
+/// screens that select these lists.
+class MemoLista<T> {
+  List<Object?>? _claves;
+  List<T>? _resultado;
+
+  List<T> obtener(List<Object?> claves, List<T> Function() calcular) {
+    final anteriores = _claves;
+    final resultado = _resultado;
+    if (anteriores != null &&
+        resultado != null &&
+        anteriores.length == claves.length) {
+      var iguales = true;
+      for (var i = 0; i < claves.length; i++) {
+        if (!identical(anteriores[i], claves[i])) {
+          iguales = false;
+          break;
+        }
+      }
+      if (iguales) return resultado;
+    }
+    final nuevo = calcular();
+    _claves = List<Object?>.of(claves);
+    _resultado = nuevo;
+    return nuevo;
+  }
+}