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/lib/main.dart

@@ -8,7 +8,7 @@ void main() => runApp(MaterialApp(
       home: ChatScreen(),
     ));
 
-/// Demonstrates UxKeyboard in a chat UI:
+/// Demonstrates XKeyboard in a chat UI:
 /// - Frame-accurate keyboard height tracking (no Flutter viewInsets lag)
 /// - Interactive dismiss (swipe the keyboard down like iMessage)
 /// - Scroll freeze while the user is panning the keyboard
@@ -20,7 +20,7 @@ class ChatScreen extends StatefulWidget {
 }
 
 class _ChatScreenState extends State<ChatScreen> {
-  final _keyboard = UxKeyboard.instance;
+  final _keyboard = XKeyboard.instance;
   final _textController = TextEditingController();
   final _messages = List.generate(
     30,
@@ -54,8 +54,8 @@ class _ChatScreenState extends State<ChatScreen> {
     // Disable Flutter's built-in resize — we handle it ourselves.
     return Scaffold(
       resizeToAvoidBottomInset: false,
-      appBar: AppBar(title: Text('UxKeyboard Chat')),
-      // ListenableBuilder rebuilds only when UxKeyboard notifies (height changes).
+      appBar: AppBar(title: Text('XKeyboard Chat')),
+      // ListenableBuilder rebuilds only when XKeyboard notifies (height changes).
       body: ListenableBuilder(
         listenable: _keyboard,
         builder: (context, _) {