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 {

android/src/main/kotlin/io/swipelab/ux/GalleryPlugin.kt

@@ -26,7 +26,7 @@ import java.io.ByteArrayOutputStream
 import java.io.File
 import java.io.FileOutputStream
 
-/// `MediaStore` bridge for `UxGallery` — paginated photo + video
+/// `MediaStore` bridge for `XGallery` — paginated photo + video
 /// queries via `ContentResolver`, cell-sized thumbnails via
 /// `ContentResolver.loadThumbnail` (API 29+), and stream-copy file
 /// resolution into the app cache so `dart:io` can read what the

android/src/main/kotlin/io/swipelab/ux/UrlPlugin.kt

@@ -14,7 +14,7 @@ class UrlPlugin : NativePlugin, MethodChannel.MethodCallHandler {
         init {
             // Trigger JNI_OnLoad in libux.so so the FFI shim's
             // android.util.Patterns bindings are cached before the first
-            // UxUrl.match call from Dart.
+            // XUrl.match call from Dart.
             System.loadLibrary("ux")
         }
     }

android/src/main/kotlin/io/swipelab/ux/XPlugin.kt

@@ -5,7 +5,7 @@ import io.flutter.embedding.engine.plugins.activity.ActivityAware
 import io.flutter.embedding.engine.plugins.activity.ActivityPluginBinding
 import io.swipelab.ux.camera.CameraPlugin
 
-class UxPlugin : FlutterPlugin, ActivityAware {
+class XPlugin : FlutterPlugin, ActivityAware {
     private val plugins: List<NativePlugin> = listOf(
         KeyboardPlugin(),
         SensorPlugin(),

android/src/main/kotlin/io/swipelab/ux/camera/CameraInstance.kt

@@ -13,7 +13,7 @@ import androidx.lifecycle.LifecycleEventObserver
 import androidx.lifecycle.ProcessLifecycleOwner
 import io.flutter.view.TextureRegistry
 
-/// One per `UxCameraController` on the Dart side. Owns CameraX's
+/// One per `XCameraController` on the Dart side. Owns CameraX's
 /// [ProcessCameraProvider] binding, the [PreviewSink] (texture +
 /// SurfaceProvider), the [PhotoCapture] use-case, and the per-instance
 /// [CustomLifecycleOwner] that drives them.

android/src/main/kotlin/io/swipelab/ux/camera/VideoCapture.kt

@@ -32,7 +32,7 @@ class VideoCapture {
     val useCase: androidx.camera.video.VideoCapture<Recorder> =
         androidx.camera.video.VideoCapture.Builder(recorder)
             // Raw sensor capture — selfie videos are "as others see
-            // you"; the preview-only mirror lives in UxCameraPreview.
+            // you"; the preview-only mirror lives in XCameraPreview.
             .setMirrorMode(MirrorMode.MIRROR_MODE_OFF)
             .build()