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.

lib/src/clipboard.dart

@@ -3,8 +3,8 @@ import 'package:flutter/services.dart';
 /// OS clipboard access for shapes Flutter's [Clipboard] doesn't cover.
 /// Right now this is image bytes — the system text path is already
 /// handled by the SDK's `Clipboard.getData(Clipboard.kTextPlain)`.
-class UxClipboard {
-  UxClipboard._();
+class XClipboard {
+  XClipboard._();
 
   static const _channel = MethodChannel('ux/clipboard');