Native (iOS + Android) now emits onViewDisposed before tearing down a
platform view's method channel. The Dart bridge resets its readiness
guard back to initializing, so sendWhenReady() queues messages until
the next UnityView attaches instead of throwing MissingPluginException
on the dead com.unity_kit/unity_view_N channel.

Also cancels the previous platform event subscription on re-initialize()
to prevent duplicated events after unload().