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/NativePlugin.swift

@@ -5,7 +5,7 @@ public protocol NativePlugin {
     func register(with registrar: FlutterPluginRegistrar)
 }
 
-public enum UxWindow {
+public enum XWindow {
     public static var keyWindow: NSWindow? {
         NSApp.keyWindow ?? NSApp.mainWindow
     }