chore(mix): remove deprecated APIs

[leoafarias]

Related issue

N/A

Description

Removes deprecated API surface from Mix and aligns tests/docs with the current v2 patterns.

Changes

• Removed deprecated utilities, variant helpers, and old API entry points across packages/mix.
• Updated spec/style/widget files and exports to reflect the simplified API footprint.
• Removed legacy tests tied to removed APIs (including variant_util_test.dart and enum_util_test.dart) and updated remaining tests.
• Updated migration/overview docs and README content to guide users to the current APIs.

Review Checklist

• Testing: Have you tested your changes, including unit tests and integration tests for affected code?
• Breaking Changes: Does this change introduce breaking changes affecting existing code or users?
• Documentation Updates: Are all relevant documentation files (e.g. README, API docs) updated to reflect the changes in this PR?
• Website Updates: Is the website containing the updates you make on documentation?

Additional Information (optional)

Verified with melos run ci and melos run analyze:dart on this branch.

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Project	Deployment	Actions	Updated (UTC)
mix-docs	Ready	Preview, Comment	Feb 27, 2026 4:03pm