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/Camera/AVCaptureConnection+iOS.swift

@@ -8,7 +8,7 @@ import AVFoundation
 /// the result. The macOS counterpart in
 /// `macos/Classes/Camera/AVCaptureConnection+macOS.swift` is a no-op.
 extension AVCaptureConnection {
-    func applyUxCaptureOrientation(_ orientation: DeviceOrientationFlutter) {
+    func applyXCaptureOrientation(_ orientation: DeviceOrientationFlutter) {
         if isVideoOrientationSupported {
             videoOrientation = orientation.avVideoOrientation
         }