fix(devices): cache platform device names, dedupe placeholder ids, purge collided EQ entries

Dart half of bt-device-identity. EstadoEcualizador now caches each
device's platform-reported name in memory so the settings screen
shows the device's own Bluetooth name instead of its raw id when no
custom rename exists, and skips auto-creating preset entries for the
composite-placeholder sentinel. Enabling multi-device EQ triggers the
Bluetooth permission request through the new channel contract. A
flag-guarded one-time migration purges only entries keyed by the
exact literal placeholder id from the three per-device preference
maps, since those collided entries cannot be attributed to a device.

Work unit 2/2 of bt-device-identity (Dart state + migration).

lib/servicios/servicio_ecualizador.dart

@@ -43,6 +43,13 @@ class ServicioEcualizador {
   static const _keyPresetsMatriz = 'eq_presets_matriz_v1';
   static const _keyNombresDispositivos = 'eq_nombres_dispositivos_v1';
 
+  /// bt-device-identity ADR-5: guard flag for the one-time placeholder purge.
+  static const _keyPlaceholderPurgaHecha = 'eq_placeholder_purge_done_v1';
+
+  /// bt-device-identity ADR-3: the exact OS placeholder MAC, in the
+  /// `bt_a2dp:`-prefixed id shape used across all device-keyed SP maps.
+  static const _placeholderMacLiteral = 'bt_a2dp:02:00:00:00:00:00';
+
   final SharedPreferences? _prefs;
 
   /// Injected startup instance (S3-R4); getInstance() is only a fallback.
@@ -50,6 +57,7 @@ class ServicioEcualizador {
       _prefs ?? SharedPreferences.getInstance();
 
   Future<ConfiguracionEcualizador> cargar() async {
+    await migrarClavesPlaceholder();
     final prefs = await _resolverPrefs();
     final principal = _leerPresetPrincipal(prefs);
     final porEmisora = _leerPresetsPorEmisora(prefs);
@@ -67,6 +75,56 @@ class ServicioEcualizador {
     );
   }
 
+  /// One-time guarded migration (bt-device-identity ADR-5): purges entries
+  /// keyed by the exact OS placeholder MAC ([_placeholderMacLiteral]) from
+  /// all three device-keyed SP maps. Idempotent — short-circuits via
+  /// [_keyPlaceholderPurgaHecha] after the first successful run. Every other
+  /// entry (including near-miss keys) is preserved byte-for-byte.
+  Future<void> migrarClavesPlaceholder() async {
+    final prefs = await _resolverPrefs();
+    if (prefs.getBool(_keyPlaceholderPurgaHecha) ?? false) return;
+
+    final presetsPorDispositivo = _leerMapa(prefs, _keyPresetsPorDispositivo);
+    if (presetsPorDispositivo.remove(_placeholderMacLiteral) != null) {
+      await _guardarMapa(
+        prefs,
+        _keyPresetsPorDispositivo,
+        presetsPorDispositivo,
+      );
+    }
+
+    // Matrix keys are "stationUuid:deviceId"; split on the FIRST colon only
+    // (station UUIDs are RFC4122 and contain no colons — multi-device-eq
+    // ADR-3), since deviceId itself may contain colons (e.g. a MAC-based id).
+    final presetsMatriz = _leerMapa(prefs, _keyPresetsMatriz);
+    final clavesMatrizAPurgar = presetsMatriz.keys.where((clave) {
+      final separador = clave.indexOf(':');
+      if (separador == -1) return false;
+      final deviceIdSegmento = clave.substring(separador + 1);
+      return deviceIdSegmento == _placeholderMacLiteral;
+    }).toList();
+    if (clavesMatrizAPurgar.isNotEmpty) {
+      for (final clave in clavesMatrizAPurgar) {
+        presetsMatriz.remove(clave);
+      }
+      await _guardarMapa(prefs, _keyPresetsMatriz, presetsMatriz);
+    }
+
+    final nombresDispositivos = _leerMapaStrings(
+      prefs,
+      _keyNombresDispositivos,
+    );
+    if (nombresDispositivos.remove(_placeholderMacLiteral) != null) {
+      await _guardarMapaStrings(
+        prefs,
+        _keyNombresDispositivos,
+        nombresDispositivos,
+      );
+    }
+
+    await prefs.setBool(_keyPlaceholderPurgaHecha, true);
+  }
+
   Future<void> guardarPrincipal(PresetEcualizador preset) async {
     final prefs = await _resolverPrefs();
     await prefs.setString(_keyPresetPrincipal, jsonEncode(preset.toJson()));