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.

pubspec.yaml

@@ -3,7 +3,7 @@ description: >-
   Flutter toolkit for fluid, native-feeling UIs. Includes frame-accurate
   keyboard height tracking via FFI with interactive dismiss, bezier utilities,
   and layout primitives.
-version: 0.9.0
+version: 0.10.0
 homepage: https://swipelab.co/ux.html
 repository: https://github.com/swipelab/ux
 issue_tracker: https://github.com/swipelab/ux/issues
@@ -33,9 +33,9 @@ flutter:
   plugin:
     platforms:
       ios:
-        pluginClass: UxPlugin
+        pluginClass: XPlugin
       android:
         package: io.swipelab.ux
-        pluginClass: UxPlugin
+        pluginClass: XPlugin
       macos:
-        pluginClass: UxPlugin
+        pluginClass: XPlugin