`ux/notifications/events` and `ux/window/events` only had macOS stream
handlers, so on Android/iOS the unconditional Dart subscription threw
MissingPluginException at startup (EventChannel reports activation
failures straight to FlutterError.onError, bypassing the `onError:`
callback).
- Gate each Dart event-channel subscription to platforms that register a
native handler (`defaultTargetPlatform`), silencing iOS.
- `WindowPlugin`: report app foreground/background as host focus via
`ProcessLifecycleOwner` ON_START/ON_STOP, so a backgrounded-but-alive
process reports `focused = false`.
- `NotificationsPlugin`: local notifications (show/cancel by thread/all),
POST_NOTIFICATIONS request on 13+, and tap routing back over the event
channel — a tap that cold-starts the process is buffered until Dart
subscribes.
- Regression tests for the subscription gate plus contract tests for the
method/event payloads.
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