shutdown method
Tear down the platform plugin. Called by Sankofa.dispose();
hosts almost never need this directly.
Implementation
Future<void> shutdown() async {
_autoConfirmTimer?.cancel();
_autoConfirmTimer = null;
// Drop the singleton unconditionally. On iOS / plain `flutter run` the
// platform plugin never came up (_ready stays false), but the instance
// is still published — an early return here would leave a disposed
// Sankofa pointing at a stale deploy.
_instance = null;
if (!_ready) return;
await SankofaDeployPlatform.instance.shutdown();
_ready = false;
}