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.

test/crash_test.dart

@@ -50,7 +50,7 @@ void main() {
       return null;
     });
 
-    await UxCrash.drainAndReport();
+    await XCrash.drainAndReport();
 
     final records = sink.snapshot();
     expect(records.length, 2);
@@ -78,7 +78,7 @@ void main() {
       throw MissingPluginException();
     });
 
-    await UxCrash.drainAndReport();
+    await XCrash.drainAndReport();
 
     expect(sink.snapshot(), isEmpty);
   });
@@ -91,7 +91,7 @@ void main() {
       throw PlatformException(code: 'oops');
     });
 
-    await UxCrash.drainAndReport();
+    await XCrash.drainAndReport();
 
     expect(sink.snapshot().length, 1);
     expect(sink.snapshot().single.level, LogLevel.warn);