Clamp simulcast lower layers to top layer (#1084)

## Summary
- Port Swift SDK simulcast lower-layer clamping behavior to Flutter.
- Clamp lower-layer max framerate so it does not exceed the configured
top layer.
- Clamp lower-layer bitrate only when the layer does not actually
downscale resolution.
- Use the larger output dimension when computing simulcast scale-down
values.
- Add coverage for clamp behavior, ladder length, unchanged presets,
priority preservation, and computed encodings.

## Testing
- `dart format --output=none --set-exit-if-changed
lib/src/types/video_parameters.dart lib/src/utils.dart
test/utils_test.dart`
- `flutter test test/utils_test.dart`
- `flutter analyze --no-pub lib/src/types/video_parameters.dart
lib/src/utils.dart test/utils_test.dart`