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.

macos/Classes/XPlugin.swift

@@ -0,0 +1,23 @@
+import FlutterMacOS
+import AppKit
+
+public class XPlugin: NSObject, FlutterPlugin {
+    private static var plugins: [NativePlugin] = []
+
+    public static func register(with registrar: FlutterPluginRegistrar) {
+        plugins = [
+            FilePlugin(),
+            ClipboardPlugin(),
+            GalleryPlugin(),
+            CameraPlugin(),
+            UrlPlugin(),
+        ]
+        for plugin in plugins {
+            plugin.register(with: registrar)
+        }
+    }
+
+    public func handle(_ call: FlutterMethodCall, result: @escaping FlutterResult) {
+        result(FlutterMethodNotImplemented)
+    }
+}