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).

test/helpers/fakes.dart

@@ -438,6 +438,9 @@ class FakeServicioDispositivoAudioThrows extends ServicioDispositivoAudio {
     throw Exception('Platform channel error: device unavailable');
   }
 
+  @override
+  Future<bool> solicitarPermisoBluetooth() async => true;
+
   @override
   Future<void> dispose() async {
     await _controller.close();
@@ -449,6 +452,16 @@ class FakeServicioDispositivoAudioThrows extends ServicioDispositivoAudio {
 /// Use [emitirDispositivo] to push device change events without needing
 /// a real platform channel.
 class FakeServicioDispositivoAudio extends ServicioDispositivoAudio {
+  FakeServicioDispositivoAudio({this.permisoBluetoothConcedido = true});
+
+  /// Value returned by [solicitarPermisoBluetooth] (bt-device-identity
+  /// Phase 2/4 testing strategy — defaults to granted so existing tests
+  /// that don't care about the permission flow are unaffected).
+  final bool permisoBluetoothConcedido;
+
+  /// Number of times [solicitarPermisoBluetooth] was called.
+  int solicitarPermisoBluetoothCalls = 0;
+
   final _controller = StreamController<DispositivoAudio>.broadcast();
   DispositivoAudio? _dispositivoActual;
 
@@ -468,6 +481,12 @@ class FakeServicioDispositivoAudio extends ServicioDispositivoAudio {
         );
   }
 
+  @override
+  Future<bool> solicitarPermisoBluetooth() async {
+    solicitarPermisoBluetoothCalls++;
+    return permisoBluetoothConcedido;
+  }
+
   /// Helper for tests: push a device change event.
   void emitirDispositivo(DispositivoAudio dispositivo) {
     _dispositivoActual = dispositivo;