onDeepLinkHandled

Clears the buffered deep link once its collector has applied it to the backstack.

_navigationDeepLink replays its last value so a deep link emitted before the collector subscribes (e.g. on cold start) isn't lost. But this ViewModel is Activity-scoped and survives configuration changes, while the collecting LaunchedEffect does not — without this, a device rotation after a deep link re-subscribes and replays the same value, duplicate-appending it onto MultiBackstack's current tab.