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/scanner.dart

@@ -12,8 +12,8 @@ import 'package:flutter/widgets.dart';
 enum BarcodeFormat { qr }
 
 /// Static helpers exposed by the platform-side scanner plugin.
-class UxScannerPermission {
-  UxScannerPermission._();
+class XScannerPermission {
+  XScannerPermission._();
 
   static const _channel = MethodChannel('ux/scanner');
 
@@ -41,8 +41,8 @@ class UxScannerPermission {
 /// Camera permission must be granted by the host app before mounting.
 /// On platforms other than iOS / Android the widget renders an empty
 /// box.
-class UxScanner extends StatefulWidget {
-  const UxScanner({
+class XScanner extends StatefulWidget {
+  const XScanner({
     super.key,
     required this.onCode,
     this.formats = const [BarcodeFormat.qr],
@@ -52,10 +52,10 @@ class UxScanner extends StatefulWidget {
   final List<BarcodeFormat> formats;
 
   @override
-  State<UxScanner> createState() => _UxScannerState();
+  State<XScanner> createState() => _XScannerState();
 }
 
-class _UxScannerState extends State<UxScanner> {
+class _XScannerState extends State<XScanner> {
   static const _events = EventChannel('ux/scanner/events');
   StreamSubscription<dynamic>? _sub;