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/servicios/servicio_dispositivo_audio_test.dart

@@ -78,5 +78,17 @@ void main() {
     test('dispose closes the stream without error', () async {
       await expectLater(fake.dispose(), completes);
     });
+
+    // bt-device-identity Task 2.5: interface-completeness — every concrete
+    // ServicioDispositivoAudio must implement solicitarPermisoBluetooth.
+    test(
+      'solicitarPermisoBluetooth increments the call counter and returns '
+      'the configured value',
+      () async {
+        final granted = await fake.solicitarPermisoBluetooth();
+        expect(granted, isTrue);
+        expect(fake.solicitarPermisoBluetoothCalls, equals(1));
+      },
+    );
   });
 }