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.

test/ux_test.dart

@@ -2,15 +2,15 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:ux/ux.dart';
 
 void main() {
-  test('UxKeyboard.instance is a singleton', () {
-    expect(UxKeyboard.instance, same(UxKeyboard.instance));
+  test('XKeyboard.instance is a singleton', () {
+    expect(XKeyboard.instance, same(XKeyboard.instance));
   });
 
-  test('UxKeyboard.height starts at 0', () {
-    expect(UxKeyboard.instance.height, 0);
+  test('XKeyboard.height starts at 0', () {
+    expect(XKeyboard.instance.height, 0);
   });
 
-  test('UxKeyboard.isOpen is false when height is 0', () {
-    expect(UxKeyboard.instance.isOpen, false);
+  test('XKeyboard.isOpen is false when height is 0', () {
+    expect(XKeyboard.instance.isOpen, false);
   });
 }