How to merge Style from parent spec.

### Use case

We have Spec
```dart
final class ButtonSpec extends Spec
with Diagnosticable, _$ButtonSpecMethods {
@override
final StyleSpec? container;

@override
final StyleSpec? icon;

@override
final StyleSpec? label;

const ButtonSpec({
this.container,
this.icon,
this.label,
});
}
```

The button widget，with `Widget child` and `Widget icon`
```
class UIButton extends StatelessWidget {
const UIButton({
super.key,
required this.onPressed,
this.child,
this.icon,
this.style,
this.variant = ButtonVariant.primary,
this.size = ButtonVariantSize.sm,
});

final VoidCallback onPressed;

final Widget? child;

final Widget? icon;

final ButtonStyler? style;

final ButtonVariant variant;

final ButtonVariantSize size;

@override
Widget build(BuildContext context) {
final child = this.child;
final icon = this.icon;

return StyleBuilder(
style: _getButtonStyle(
style,
variant: variant,
size: size,
),
builder: (context, spec) => FlexBox(
styleSpec: spec.container,
children: [
if (spec.label != null && child != null)
StyleSpecProvider(
spec: spec.label!,
child: child,
),
if (spec.icon != null && icon != null)
StyleSpecProvider(
spec: spec.icon!,
child: icon,
),
],
),
);
}
}
```

### Proposal

What we want to do is:
```
UIButton(
onPressed: () {},
size: size,
variant: variant,
child: StyledText(
title,
styleSpec: StyleSpecProvider.of(context),
style: TextStyler().fontSize(38) // <= merge context style，and make font bigger only
),
);
```

Start by tracing how StyleSpecProvider.of(context), StyledText, and TextStyler currently obtain and apply styles. Define how a child style should merge with the parent context style, including the font-size override shown in the proposal; done means the intended UIButton and StyledText usage works consistently.

Written by the indexing model from the issue text.