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.

darwin/Camera/CameraInstance.swift

@@ -6,7 +6,7 @@ import FlutterMacOS
 #endif
 import Foundation
 
-/// One per `UxCameraController` on the Dart side. Owns its
+/// One per `XCameraController` on the Dart side. Owns its
 /// `AVCaptureSession`, the texture-backed preview pipeline, photo
 /// output, audio + video data outputs, and the
 /// [VideoRecorder] when a recording is in flight. Multiple instances
@@ -526,7 +526,7 @@ final class CameraInstance {
             // front camera — raw sensor feed at capture, mirror as a
             // playback decision.
             if let videoConn = videoDataOutput?.connection(with: .video) {
-                videoConn.applyUxCaptureOrientation(
+                videoConn.applyXCaptureOrientation(
                     lockedOrientation ?? orientation.current
                 )
                 if videoConn.isVideoMirroringSupported {
@@ -558,7 +558,7 @@ final class CameraInstance {
 
     private func applyVideoOrientationOnPreview(_ next: DeviceOrientationFlutter) {
         videoDataOutput?.connection(with: .video)?
-            .applyUxCaptureOrientation(next)
+            .applyXCaptureOrientation(next)
     }
 
     private func emit(_ extras: [String: Any]) {

darwin/Camera/CaptureDevice.swift

@@ -108,7 +108,7 @@ enum CaptureDevice {
 
 /// One row of the discovery result. `lens` and `sensorOrientation`
 /// match the wire shape expected by Dart's
-/// `MethodChannelUxCameraBackend.availableCameras()`.
+/// `MethodChannelXCameraBackend.availableCameras()`.
 struct DiscoveredCamera {
     let device: AVCaptureDevice
     let lens: String

darwin/Camera/DeviceOrientation.swift

@@ -18,7 +18,7 @@ public enum DeviceOrientationFlutter: String {
     case landscapeRight
 
     /// Parse a wire string. Returns `.portraitUp` for unknown inputs
-    /// (matches the Dart-side fallback in `MethodChannelUxCameraBackend`).
+    /// (matches the Dart-side fallback in `MethodChannelXCameraBackend`).
     public static func parse(_ raw: String?) -> DeviceOrientationFlutter {
         return DeviceOrientationFlutter(rawValue: raw ?? "") ?? .portraitUp
     }

darwin/Camera/PhotoOutput.swift

@@ -49,7 +49,7 @@ final class PhotoOutput {
         // JPEG); macOS is a no-op (desktop cams are physically
         // landscape, any rotation skews the photo). See
         // `AVCaptureConnection+iOS.swift` / `…+macOS.swift`.
-        connection.applyUxCaptureOrientation(orientation)
+        connection.applyXCaptureOrientation(orientation)
         // The recorded photo carries no mirror; mirroring is a
         // preview-only concern.
         if connection.isVideoMirroringSupported {
@@ -79,7 +79,7 @@ final class PhotoOutput {
             // snapshot defaults cleanly. No-op on macOS (the
             // extension method is empty there).
             self?.avOutput.connection(with: .video)?
-                .applyUxCaptureOrientation(.portraitUp)
+                .applyXCaptureOrientation(.portraitUp)
             self?.inFlight = nil
             DispatchQueue.main.async { completion(result) }
         }