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.

darwin/Camera/DeviceOrientation.swift

@@ -18,7 +18,7 @@ public enum DeviceOrientationFlutter: String {
     case landscapeRight
 
     /// Parse a wire string. Returns `.portraitUp` for unknown inputs
-    /// (matches the Dart-side fallback in `MethodChannelUxCameraBackend`).
+    /// (matches the Dart-side fallback in `MethodChannelXCameraBackend`).
     public static func parse(_ raw: String?) -> DeviceOrientationFlutter {
         return DeviceOrientationFlutter(rawValue: raw ?? "") ?? .portraitUp
     }