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.

ios/Classes/KeyboardPlugin.swift

@@ -268,7 +268,7 @@ public class KeyboardPlugin: NSObject, NativePlugin {
     }
 
     private func setupPanGesture() {
-        guard let window = UxWindow.keyWindow else { return }
+        guard let window = XWindow.keyWindow else { return }
         let view: UIView = window.rootViewController?.view ?? window
 
         let pan = UIPanGestureRecognizer(target: self, action: #selector(handlePan))