feat(mix_generator): support plain MixWidget targets

[leoafarias]

Related issue

No linked issue.

Description

Extend @MixWidget so Remix can generate wrappers around plain widget
constructors without requiring StyleWidget inheritance or a Styler extension
call() method.

Changes

• Add MixWidget.target and independent factoryParameters selection.
• Derive wrapper fields, defaults, keys, and generic parameters from the target
  constructor while excluding style and styleSpec.
• Instantiate the target directly with the recipe result passed through
  style.
• Preserve enum-derived named constructors, shared factory/target parameters,
  same-build generated Stylers, and the existing no-target path.
• Bump the coordinated prereleases to mix_annotations 2.2.0-beta.1 and
  mix_generator 2.2.0-beta.2.

Review Checklist

• Testing: Full mix_annotations and mix_generator test suites pass.
• Breaking Changes: Existing @MixWidget behavior remains unchanged
  when target is omitted.
• Documentation Updates: Annotation and generator READMEs and
  changelogs document the new API.
• Website Updates: No website update is included.

Additional Information (optional)

The Remix integration was exercised against all 25 historical Fortal wrappers:
clean generation, Flutter analysis, and 1,889 Remix tests pass using these
unpublished package versions.

Downstream integration:

• conceptadev/remix#84 replaces
  the handwritten Fortal wrappers with deterministic generator output.
• conceptadev/remix#80 is the
  visual-parity PR stacked on that generated-wrapper migration.

After this PR merges, publish the coordinated
mix_annotations 2.2.0-beta.1 and mix_generator 2.2.0-beta.2 prereleases.
Both Remix PRs intentionally remain draft until those versions resolve from
pub.dev and their CI jobs can be rerun.