pub.dev score: license, dartdoc, analysis, tests

- Add MIT license
- Add dartdoc to all public APIs (UxKeyboard, BendBox, Bezier, Json)
- Add analysis_options.yaml with flutter_lints and public_member_api_docs
- Add repository, issue_tracker, topics to pubspec.yaml
- Expand package description
- Fix BendBox constructor (const, super.key)
- Remove unused import in keyboard.dart
- Replace boilerplate tests with UxKeyboard smoke tests
- Move ux dependency to example's dependencies (not dev_dependencies)

lib/ux.dart

@@ -1,6 +1,10 @@
-library ux;
+/// Flutter toolkit for fluid, native-feeling UIs.
+///
+/// Includes [UxKeyboard] for frame-accurate keyboard height tracking,
+/// [BendBox] for curved layout painting, and bezier curve utilities.
+library;
 
 export 'src/bend_box.dart';
 export 'src/json_extension.dart';
 export 'src/bezier.dart';
-export 'src/keyboard.dart';
+export 'src/keyboard.dart';