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/anim/animated_double.dart

@@ -0,0 +1,61 @@
+import 'package:flutter/widgets.dart';
+
+class AnimatedDouble extends StatefulWidget {
+  const AnimatedDouble({
+    super.key,
+    required this.builder,
+    required this.start,
+    required this.end,
+    this.curve = Curves.linear,
+    this.duration = const Duration(milliseconds: 200),
+    this.child,
+  });
+
+  final ValueWidgetBuilder<double> builder;
+  final Widget? child;
+  final double start;
+  final double end;
+  final Duration duration;
+  final Curve curve;
+
+  @override
+  State<AnimatedDouble> createState() => _AnimatedDoubleState();
+}
+
+class _AnimatedDoubleState extends State<AnimatedDouble>
+    with SingleTickerProviderStateMixin {
+  late final AnimationController controller;
+
+  @override
+  void initState() {
+    super.initState();
+    controller = AnimationController.unbounded(
+        vsync: this, value: widget.start, duration: widget.duration);
+    if (widget.end != widget.start) {
+      controller.animateTo(widget.end);
+    }
+  }
+
+  @override
+  void dispose() {
+    controller.dispose();
+    super.dispose();
+  }
+
+  @override
+  void didUpdateWidget(covariant AnimatedDouble oldWidget) {
+    super.didUpdateWidget(oldWidget);
+    if (widget.end != oldWidget.end) {
+      controller.animateTo(widget.end);
+    }
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return ValueListenableBuilder(
+      valueListenable: controller,
+      builder: widget.builder,
+      child: widget.child,
+    );
+  }
+}