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/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"))
 }