feat: add applyVariants method for NamedVariant application

[leoafarias]

Summary

• Adds applyVariants() method to VariantStyleMixin for applying named variants to styles
• Creates internal AppliedVariantsStyle wrapper class to carry pre-applied variants

Problem

Users could create NamedVariant and add them to styles via .variant(), but there was no API to activate them (issue #794).

Solution

Adds a simple applyVariants() method that returns a wrapped style which passes the specified variants to build() during resolution.

Usage

final style = BoxStyler()
  .variant(small, BoxStyler().width(50))
  .variant(large, BoxStyler().width(200));

// Apply the small variant
Box(style: style.applyVariants([small]))

Test plan

• Added unit tests for applyVariants covering single/multiple variants, merging, and widget tree integration

Closes #794

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Project	Deployment	Review	Updated (UTC)
mix-docs	Ready	Preview, Comment	Dec 19, 2025 3:26pm

[Copilot]

Pull request overview

This PR adds an applyVariants() method to the Mix framework that allows developers to pre-apply named variants to styles, addressing issue #794 where users could define named variants but had no API to activate them.

Key Changes:

• Adds applyVariants() method to VariantStyleMixin for pre-applying named variants
• Creates internal AppliedVariantsStyle wrapper class that stores and applies variants during resolution
• Includes comprehensive test coverage for single/multiple variants, merging, and widget integration

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
packages/mix/lib/src/style/mixins/variant_style_mixin.dart	Adds the applyVariants() method with documentation and example usage
packages/mix/lib/src/core/internal/applied_variants_style.dart	Implements internal wrapper class that carries pre-applied variants and combines them with runtime variants
packages/mix/test/src/style/mixins/variant_style_mixin_test.dart	Adds comprehensive test suite covering single/multiple variants, merging, and widget tree integration

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