feat(eq): add per-device equalizer with 4-level preset resolution

Introduce multi-device EQ support allowing each audio output device
(built-in speaker, wired, USB, individual Bluetooth by MAC) to have
its own equalizer preset, combined with existing per-station presets
for a full station×device matrix.

- Add DispositivoAudio model and ServicioDispositivoAudio interface
- Add Android platform channel (AudioDeviceCallback) for device detection
- Add iOS AudioDevicesPlugin (AVAudioSession route tracking)
- Extend ServicioEcualizador with device and matrix persistence keys
- Implement 4-level resolution: matrix > station > device > global
- Add advanced EQ settings section with feature toggle (off by default)
- Extend export/import to v3 with backward compatibility
- 184 tests passing, zero analyzer issues

lib/servicios/servicio_export_import.dart

@@ -6,21 +6,29 @@ import '../modelos/preset_ecualizador.dart';
 
 /// Owns the backup (export/import) JSON serialization (S4-R4).
 ///
-/// The v2 envelope produced here is byte-compatible with the legacy format
-/// previously assembled inline by `EstadoRadio.exportarConfig` and
-/// pretty-printed by `pantalla_ajustes.dart`, so existing exports keep
-/// round-tripping. State APPLICATION (writing favorites, EQ, alarms back
-/// into the app) stays in `EstadoRadio.importarConfig` — this service only
-/// owns serialization, parsing and the envelope shape.
+/// v3 extends v2 with `presetsPorDispositivo`, `presetsMatriz`, and
+/// `eqMultiDeviceEnabled`. When those optional parameters are omitted the
+/// export stays at v2 for backward compat with the old app. State APPLICATION
+/// (writing favorites, EQ, alarms back into the app) stays in
+/// `EstadoRadio.importarConfig` — this service only owns serialization,
+/// parsing and the envelope shape.
 class ServicioExportImport {
   const ServicioExportImport();
 
-  /// Current backup schema version (v2 — full portability).
-  static const int versionActual = 2;
+  /// Current backup schema version (v3 — multi-device EQ).
+  static const int versionActual = 3;
 
-  /// Builds the v2 export envelope. Key set and semantics must stay exactly
-  /// as the legacy inline export so old backups remain importable:
-  /// the `alarmas` block is the RAW JSON map persisted by ServicioAlarmas
+  /// Legacy v2 version constant kept for clarity.
+  static const int versionV2 = 2;
+
+  /// Builds the export envelope.
+  ///
+  /// When [presetsPorDispositivo] or [presetsMatriz] are provided (non-null),
+  /// [versionActual] (3) is written. When both are omitted the call behaves
+  /// identically to the original v2 format (version key stays 2) so old
+  /// backups keep round-tripping without version bumps.
+  ///
+  /// The `alarmas` block is the RAW JSON map persisted by ServicioAlarmas
   /// and passes through untouched (no re-parsing here).
   Map<String, dynamic> construirExportacion({
     required List<GrupoFavoritos> gruposFavoritos,
@@ -33,9 +41,18 @@ class ServicioExportImport {
     required String ordenListas,
     required List<int> timerSuenoPresetsSegundos,
     DateTime? exportadoEn,
+    // v3 extensions — omitting these produces a v2-compatible export.
+    Map<String, PresetEcualizador>? presetsPorDispositivo,
+    Map<String, PresetEcualizador>? presetsMatriz,
+    bool? eqMultiDeviceEnabled,
   }) {
-    return {
-      'version': versionActual,
+    final tieneExtensionesV3 =
+        presetsPorDispositivo != null ||
+        presetsMatriz != null ||
+        eqMultiDeviceEnabled != null;
+
+    final envelope = <String, dynamic>{
+      'version': tieneExtensionesV3 ? versionActual : versionV2,
       'exportedAt': (exportadoEn ?? DateTime.now()).toIso8601String(),
       // Favorites + groups (preserves grupo_id assignments per station).
       // The protected "sin asignar" group is implicit and never exported.
@@ -47,7 +64,7 @@ class ServicioExportImport {
       'favoritos': favoritos.map((e) => e.toMap()).toList(),
       // Custom stations.
       'emisorasCustom': emisorasCustom.map((e) => e.toMap()).toList(),
-      // Equalizer.
+      // Equalizer (base fields — present in all versions).
       'presetPrincipalEcualizador': presetPrincipal.toJson(),
       'presetsEcualizador': presetsPorEmisora.map(
         (uuid, preset) => MapEntry(uuid, preset.toJson()),
@@ -59,6 +76,19 @@ class ServicioExportImport {
       'ordenListas': ordenListas,
       'timerSuenoPresetsSegundos': timerSuenoPresetsSegundos,
     };
+
+    // v3 extensions: only written when explicitly provided.
+    if (tieneExtensionesV3) {
+      envelope['presetsPorDispositivo'] = (presetsPorDispositivo ?? {}).map(
+        (deviceId, preset) => MapEntry(deviceId, preset.toJson()),
+      );
+      envelope['presetsMatriz'] = (presetsMatriz ?? {}).map(
+        (clave, preset) => MapEntry(clave, preset.toJson()),
+      );
+      envelope['eqMultiDeviceEnabled'] = eqMultiDeviceEnabled ?? false;
+    }
+
+    return envelope;
   }
 
   /// Serializes an export envelope to pretty-printed JSON (same formatting