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.

android/src/main/jni/url_detect.cpp

@@ -1,4 +1,4 @@
-// Native data detection for UxUrl on Android. Synchronous, callable via dart:ffi.
+// Native data detection for XUrl on Android. Synchronous, callable via dart:ffi.
 //
 // Exports two symbols:
 //   uint8_t* ux_match_url(const uint16_t* utf16, int32_t len, int32_t* out_size);
@@ -24,7 +24,7 @@
 #include <string>
 #include <vector>
 
-#define UX_LOG_TAG "UxUrl"
+#define UX_LOG_TAG "XUrl"
 #define UX_LOGE(...) __android_log_print(ANDROID_LOG_ERROR, UX_LOG_TAG, __VA_ARGS__)
 
 namespace {