chore(mix): prepare 2.2.0-beta.3

[leoafarias]

Summary

• bump mix from 2.2.0-beta.2 to 2.2.0-beta.3
• finalize the unreleased changelog for the focus-visible variant, Pressable behavior and semantics, nested widget-state discovery, and interaction/focus fixes

Validation

• melos run gen:build
• melos run ci
• dart analyze across all packages
• Mix DCM analysis (no issues)
• flutter pub publish --dry-run (0 warnings; expected local Melos override hint)
• Mix changelog version matches its pubspec

Known repository-wide baseline

The aggregate DCM command still reports pre-existing issues in mix_annotations and mix_generator; neither package is changed by this release PR.