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, _) {

example/macos/Flutter/GeneratedPluginRegistrant.swift

@@ -8,5 +8,5 @@ import Foundation
 import ux
 
 func RegisterGeneratedPlugins(registry: FlutterPluginRegistry) {
-  UxPlugin.register(with: registry.registrar(forPlugin: "UxPlugin"))
+  XPlugin.register(with: registry.registrar(forPlugin: "XPlugin"))
 }

example/pubspec.lock

@@ -510,7 +510,7 @@ packages:
       path: ".."
       relative: true
     source: path
-    version: "0.9.0"
+    version: "0.10.0"
   vector_math:
     dependency: transitive
     description:

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);
   });
 }