ux: bulk WIP — UxPlugin→XPlugin rename + new anim/core/navi/reactive packages

Catch-all commit for outstanding pre-existing local changes. Mixes
several themes that would normally be split:

- Rename: UxPlugin → XPlugin across iOS, macOS, Android registrants.
- New top-level packages under lib/src/: anim/ (animated values,
  panes, sheets, dock, measured), core/ (Emitter, ReactiveBuilder
  scaffolding, presenter/widget/value/dispose primitives), navi/
  (Screen/ScreenStack/Router/hero/transitions), reactive/.
- Edits across existing plugins (clipboard, crash, file, gallery,
  keyboard, scanner, sensor, url) to align with the new core.
- Test updates and CHANGELOG/README touches accompanying the above.

example/test/widget_test.dart

@@ -5,7 +5,7 @@ import 'package:ux_example/main.dart';
 void main() {
   testWidgets('ChatScreen renders', (WidgetTester tester) async {
     await tester.pumpWidget(MaterialApp(home: ChatScreen()));
-    expect(find.text('UxKeyboard Chat'), findsOneWidget);
+    expect(find.text('XKeyboard Chat'), findsOneWidget);
     expect(find.text('Type a message...'), findsOneWidget);
   });
 }