feat(mix_generator): emit styler field metadata

[leoafarias]

Related issue

Related to #1026.

Description

Expose a generated, runtime-safe inventory of Styler source fields so protocol
adapters and tooling can validate complete field coverage without maintaining a
second handwritten field manifest.

The capability lives in mix, while source discovery remains in
mix_generator; consumers do not acquire a runtime dependency on the
generator.

Changes

• Add the StylerFieldMetadata capability to mix.
• Emit const $stylerFieldNames metadata for generated Styler classes and
  legacy @MixableStyler mixins.
• Reserve metadata member names and report source-located generator errors for
  collisions.
• Keep handwritten GridBoxStyler aligned with the generated contract.
• Centralize the inherited animation, modifier, and variants field names
  across generated metadata, props, and legacy extraction.
• Derive the generated metadata getter from its reserved source name so both
  validation and emission share one spelling.
• Update generated Mix and Mix Chart output, package versions, changelogs, and
  conformance coverage.

Review Checklist

• Testing: melos run gen:build and melos run ci pass.
• Breaking Changes: No. The metadata interface is additive.
• Documentation Updates: Public Dartdoc and package changelogs are updated.
• Website Updates: Not required for generator/runtime metadata.

Additional Information (optional)

melos run analyze:dart passes across all packages. The repository-wide DCM
stage continues to report the existing unrelated findings in untouched
mix_annotations (1) and mix_generator (12) files.