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/GalleryPlugin.swift

@@ -3,7 +3,7 @@ import Photos
 import PhotosUI
 import UIKit
 
-/// `Photos.framework` bridge for `UxGallery` — paginated asset queries,
+/// `Photos.framework` bridge for `XGallery` — paginated asset queries,
 /// cell-sized thumbnails via `PHCachingImageManager`, and on-demand
 /// file resolution into the app cache.
 public class GalleryPlugin: NSObject, NativePlugin, PHPhotoLibraryChangeObserver, FlutterStreamHandler {
@@ -101,7 +101,7 @@ public class GalleryPlugin: NSObject, NativePlugin, PHPhotoLibraryChangeObserver
             // Completion handler signals dismissal; reload is driven by
             // `ux/gallery/changes` via the library observer (fires after
             // iOS commits the new subset).
-            if #available(iOS 15, *), let vc = UxWindow.topViewController {
+            if #available(iOS 15, *), let vc = XWindow.topViewController {
                 PHPhotoLibrary.shared().presentLimitedLibraryPicker(from: vc) { _ in
                     // Apple's docs: the completion handler runs on "an
                     // arbitrary serial dispatch queue". Flutter method-
@@ -110,7 +110,7 @@ public class GalleryPlugin: NSObject, NativePlugin, PHPhotoLibraryChangeObserver
                         result(nil)
                     }
                 }
-            } else if #available(iOS 14, *), let vc = UxWindow.topViewController {
+            } else if #available(iOS 14, *), let vc = XWindow.topViewController {
                 PHPhotoLibrary.shared().presentLimitedLibraryPicker(from: vc)
                 result(nil)
             } else {