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/core/widget.dart

@@ -0,0 +1,29 @@
+import 'package:flutter/widgets.dart';
+
+extension GlobalKeyExtension on GlobalKey {
+  Offset? get position {
+    final context = currentContext;
+    if (context == null) return null;
+    final box = context.findRenderObject() as RenderBox;
+    final pos = box.localToGlobal(Offset.zero);
+    return pos;
+  }
+
+  Size? get size {
+    final context = currentContext;
+    if (context == null) return null;
+    final box = context.findRenderObject() as RenderBox;
+    return box.size;
+  }
+
+  Rect? rect({Offset offset = Offset.zero}) {
+    final context = currentContext;
+    if (context == null) return null;
+    final box = context.findRenderObject() as RenderBox;
+    return box.localToGlobal(offset) & box.size;
+  }
+}
+
+extension SliverExtension on Widget {
+  Widget sliver({Key? key}) => SliverToBoxAdapter(key: key, child: this);
+}